What is Color?

Legal Definition
In United States law, the term color of law denotes the "mere semblance of legal right", the "pretense or appearance of" right; hence, an action done under color of law colors (adjusts) the law to the circumstance, yet said apparently legal action contravenes the law. Under color of authority is a legal phrase used in the US indicating that a person is claiming or implying the acts he or she is committing are related to and legitimized by his or her role as an agent of governmental power, especially if the acts are unlawful.
-- Wikipedia
Legal Definition
The appearance of a valid claim, right, or authority at a given point in time. The term is often used in hindsight when the claim, right, or authority actually proves to be invalid.

See, e.g., Color of law, Color of office, Color of title, Colorable claim, and Colorable transaction.
Legal Definition
Pleading. It is of two kinds, namely, express color, and implied color. 2. Express color. This is defined to be a feigned matter, pleaded by the defendant, in an action of trespass, from which the plaintiff seems to have a good cause of action, whereas he has in truth only an appearance or color of cause. The practice of giving express color in pleas, obtained in the mixed actions of assize, the writ of entry in the nature of assize, as well as in the personal action of trespass. Steph. on Plead. 230; Bac. Ab. Trespass, 14.

3. It is a general rule in pleading that no man shall be allowed to plead specially such plea as amounts to the general issue, or a total denial of the charges contained in the declaration, and must in such cases plead the general issue in terms, by which the whole question is referred to the jury; yet, if the defendant in an action of trespass, be desirous to refer the validity of his title to the court, rather than to the jury; he may in his plea stated his title specially, by expressly giving color of title to the plaintiff, or supposing him to have an appearance of title, had indeed in point of law, but of which the jury are not competent judges. 3 Bl. Com. 309. Suppose, for example, that the plaintiff wag in wrongful possession of the close, without any further appearance of title than the possession itself, at the time of the trespass alleged, and that the defendants, entered upon him in assertion of their title: but being unable to set forth this title in the pleading, in consequence of the objection that would arise for want of color, are driven to plead the general issue of not guilty. By this plea an issue is produced whether the defendants are-guilty or not of the trespass; but upon the trial of the issue, it will be found that the question turns entirely upon a construction of law. The defendants say they are not guilty of the trespasses, because they are not guilty of breaking the close of the plaintiff, as alleged in the declaration; and that they are not guilty of breaking the close of the plaintiff, because they themselves had the property in that close; and their title is. this, that the father of one of the defendants being seised of the close in fee, gave it in tail to his eldest son, remainder in tail to one of the defendants; the eldest son was disseised, but made continual claim till the death of the disseisor; after whose death, the descent being cast upon the heir, the disseisee entered upon the heir, and afterwards died, when the remainder took effect in the said defendant who demised to the other defendant . Now, this title involves a legal question; namely, whether continual claim will no preserve the right of entry in the disseisee, notwithstanding a descent cast on the heir of the disseisor. (See as to this point, Continual Claim.) The issue however is merely not guilty, and this is triable by jury; and the effect, therefore, would be, that a jury would have to decide this question of law, subject to the direction upon it, which they would receive from the court. But, let it be supposed that the defendants, in a view to the more satisfactory decision of the question, wish to bring it under the consideration of the court in bank, rather than have it referred to a jury. If they have any means of setting forth their title specially in the plea, the object will be attained; for then the plaintiff, if disposed to question the sufficiently of the title, may demur to the plea, and thus refer the question to the decision of the judges. But such plea if pleaded simply, according to the state of the fact, would be informal for want of color; and hence arises a difficulty.

4. The pleaders of former days, contrived to overcome this difficulty in the following singular manner. In such case as that supposed, the plea wanting implied color, they gave in lieu of it an express one, by inserting a fictitious allegation of some colorable title in the plaintiff, which they, at the same time avoided by the preferable title of the defendant. S Step . Pl. 225 Brown's Entr. 343, for a form of the plea. Plowd. Rep. 22 b.

5. Formerly various suggestions of apparent right, might be adopted according to the fancy of the pleader; and though the same latitude is, perhaps, still available, yet, in practice, it is unusual to resort to any except certain known fictions, which long usage has applied to the particular case for example, in trespass to land, the color universally given is that of a defective charter of the demise. See, in general, 2 Saund. 410; 10 Co. 88; Cro. Eliz. 76; 1 East, 215; Doct. Pl. 17; Doct. & Stud. lib. 2, c. 53; Bac. Abr. Pleas, I 8; Trespass, I 4; 1 Chit. Pl. 500 Steph. on Pl. 220.

6. Implied color. That in pleading which admits by implication, an apparent right in the opposite party, and avoids it by pleading some new matter by which that apparent right is defeated. Steph. Pl. 225.

7. It is a rule that every pleading by way of confession and avoidance, must give color; that is, it must admit an apparent right in the opposite party, and rely, therefore, on some new matter by which that apparent right is defeated. For example, where the defendant pleads a release to an action for breach of covenant, the tendency of the plea is to admit an apparent right in the plaintiff, namely, that the defendant did, as alleged in the declaration, execute the deed and break the covenant therein contained, and would therefore, prima facie, be liable on that ground; but shows new matter not before disclosed, by which that apparent right is done away, namely, that the plaintiff executed to him a release. Again, if the plaintiff reply that Such release was obtained by duress, in his, replication, he impliedly admits that the defendant has, prima facie, a good defence, namely, that such release was executed as alleged in the plea; and that the defefadant therefore would be discharged; but relies on new matter by which the plea is avoided, namely, that the release was obtained by duress. The plea, in this case, therefore, gives color to the declaration, and the replication, to the plea. But let it be supposed that the plaintiff has replied, that the release was executed by him, but to another person, and not to the defendant; this would be an informal replication wanting color; because, if the release were not to the defendant there would not exist even an apparent defence, requiring the allegation of new matter to avoid it, and the plea might be sufficiently answered by a traverse, denying that the deed stated in the plea is the deed of the plaintiff. See Steph. Pl. 220; 1 Chit. Pl. 498; Lawes, Civ. Pl. 126; Arch. Pl. 211; Doct. Pl. 17; 4 Vin. Abr. 552; Bac. Abr. Pleas, &e. I 8; Com. Dig. Pleader, 3 M 40, 3-M 41. See an example of giving color in pleading in the Roman law, Inst. lib. 4, tit 14, De replicantionibus.
-- Bouviers Law Dictionary
Legal Definition
An appearance, semblance or simulacrum, as distinguished from that which is real. A prima facie or apparent right Hence, a deceptive appearance; a plausible, assumed exterior, concealing a lack of reality ; a flisguise or pretext. Railroad Co. v. Allfree, 64 Iowa, 500, 20 N. W. 779; Berks County v. Railroad Co., 167 Pa. 102, 31 Atl. 474; Broughton v. Haywood, 61 N. C. 383.

In pleading. Ground of action admitted to subsist in the opposite party by the pleading of one of the parties to an action, which is so set out as to be apparently valid, but which is in reality legally insufficient. This was a term of the ancient rhetoricians, and early adopted into the language of pleading. It was an apparent or prima facie right; and the meaning of the rule that pleadings in confession and avoidance should give color was that they should confess the matter adversely alleged, to such an extent, at least, as to admit some apparent right in the opposite party, which required to be encountered and avoided by the allegation of new matter. Color was either express, i. e., inserted in the pleading, or implied, which was naturally inherent in the structure of the pleading. Steph. PI. 233; Merten v. Bank, 5 Okl. 585, 49 Pac. 913.

The word also means the dark color of the skin showing the presence of negro blood; and hence it is equivalent to African descent or parentage.
-- Black's Law Dictionary
Legal Definition
Appearance of right.
-- Ballentine's Law Dictionary