What colour is a neuromodulator?

There is no right answer. Colours in figures are assigned by whoever made the figure, so what you are reporting is your own association, not a fact about the molecule. Answer with whatever comes first.

If serotonin were a colour, which would it be?
If noradrenaline were a colour, which would it be?
If dopamine were a colour, which would it be?
If acetylcholine were a colour, which would it be?
Do you work in neuroscience?