not necessarily colourful (got to suit the horse, can't blend in too much or clash with the horse's coat)... but tack has to match, depending on your discipline. I like it to match anyway. I had a pony that I never showed even once and it drove me mad that his bridle was brown but his saddle was black. I always did think brown looked nicer on him (he was grey) but it was too expensive to go buy another saddle just because the one we had was the wrong colour.
Most of my horses' stuff does match, Satin's stuff is mostly blue and Monty's stuff is mostly purple (should it be the other way round?? LOL), but that's so I know whose brush/bucket/halter and lead is whose. Not that halters get confused, Satin is maybe pony sized and Monty is in full size for nearly everything.
Of course I have photos somewhere of me and Monty in the ULTIMATE fashion fail. Me mostly in black with a bit of gold on my t-shirt, and then him in a purple PVC bridle, maroon saddle pad, day-glo yellow quartersheet... yeah can you say clash? LOL