14 CFR § 45.21

Except as provided in § 45.22, no person may operate a U.S.-registered aircraft unless that aircraft displays nationality and registration marks in accordance with the requirements of this section and §§ 45.23 through 45.33.

Unless otherwise authorized by the FAA, no person may place on any aircraft a design, mark, or symbol that modifies or confuses the nationality and registration marks.

Aircraft nationality and registration marks must—

Except as provided in paragraph (d) of this section, be painted on the aircraft or affixed by any other means insuring a similar degree of permanence;

Have no ornamentation;

Contrast in color with the background; and

Be legible.

The aircraft nationality and registration marks may be affixed to an aircraft with readily removable material if—

It is intended for immediate delivery to a foreign purchaser;

It is bearing a temporary registration number; or

It is marked temporarily to meet the requirements of § 45.22(c)(1) or § 45.29(h) of this part, or both.