The FRA build rules say

Power Light
All Featherweights, Lightweights, Middleweights, Heavyweights and Super Heavy Weights, must have at
least one surface mounted non-filament power light that is illuminated when power is supplied to the robot
(i.e. when the link is in). The power light can be any colour but must be non-flashing and be in contrast with
it's surroundings.

I use a 24v LED from Technobots the same type in all my robots Heavy and feather so if one
breaks the spare I carry fits all.
If you keep it near the link it makes it easier to see during activation and easier to wire up.