My bathroom doesn't have much colour in it, and as there's so much colour around the rest of the house I like it that way.
But it could remain a pale, calming space and still handle a cheer-up with one of these sunshine yellow bath mats by HAY.
£18 from Clippings
And if you're feeling yellow even more than this, these anaglypta-ish bathing brights might appeal too (and if you can't find those locally, HAY also do some lovely egg-yolk yellow and white towels available at Heal's among other shops). Or perhaps you're bold enough for an entire yellow bathroom...