The colour of the light emitted emitted by GTLS (Gaseous Tritium Light Source) devices is determined by the phosphur used to coat the inner surface of the glass tube. The resulting colour can be any specified colour of the visual spectrum but the human eye's perception in darkness has maximum sensitivity to green and yellow. For this reason, GTLS sources in green are the most usual colour used with luminous watch dials etc. The following table illustrates comparisons:

Colour Green Yellow Orange Blue Red White
Intensity Relative To Green 100% 85% 60% 25% 25% 60%
Approx. Representation

Where a GTLS device requires the use of two colours, green and orange are usually paired for their easily recognised contrast.

Although red and blue are relatively weaker (25% of the perceived brightness of green), they have advantages in situations requiring undisturbed night vision of the eye (i.e. dark adaption) - for example with astrononomers.

Self-illuminated watches used by professional and recreational divers at greater depths tend to have blue (rather than green) GTLS sources because blue is the only (last) colour still visible beyond a certain depth due to light refraction under water.