The UEFA administration measures the contrast of colours used on playing attire using a spectrophotometer to provide an objective means of decision-making.
Measurements are taken using a spherical spectrophotometer with an aperture of 6.6mm and a measurement area of 4mm on a white-point of D65 and a Standard Observers Angle of 10°. A spectrally neutral 18% Reflectance Grey Card is placed under the single layer of fabric to be measured, and each final measurement is calculated from the average of three readings, at 0°, 90° and 45°.
The contrast between shirt colours in the number zone must be less than or equal to a Delta L of 25. Colour combinations with a Delta L difference value greater than 25 require a single-colour patch on the back of the shirt to serve as the number zone.
Player numbers must contrast with the background colour(s) in the number zone by a Delta L value equal to or higher than 30.
Tonal prints must be included in a tonal variation of the (background) colour(s) of the item of playing attire into which they are incorporated, with a contrast that is less than or equal to a Delta L value of 10. Background colours are measured before the associated tonal print.
Jacquard weave thread must be included in a thread that must be the same colour as the fabric into which it is woven. The jacquard weave must not dominate, contain a colour that contrasts with, or otherwise affect the distinctiveness of the item of playing attire into which it is incorporated. When measured, the contrast between the two weaves must be less than or equal to a Delta L value of 5. Background colours are measured before the associated jacquard weave thread.