14 CFR § 25.1397

Each position light color must have the applicable International Commission on Illumination chromaticity coordinates as follows:

y is not greater than 0.335; and

z is not greater than 0.002.

x is not greater than 0.440−0.320y ;

x is not greater than y−0.170; and

y is not less than 0.390−0.170x.

x is not less than 0.300 and not greater than 0.540;

y is not less than x−0.040; or y0−0.010, whichever is the smaller; and

y is not greater than x + 0.020 nor 0.636−0.400x;

Where y0 is the y coordinate of the Planckian radiator for the value of x considered.