Since the 1970s around here coloured sails have been popular with the fun boats like off the beach catamarans (Hobie Cat and similar).
We occasionally see wonderous rainbow sail colour combinations.
However I suspect that a sailmaker can only buy coloured material in whole roll lots and if you consider the variety of colours, materials and weights. It is hardly surprising that they steer us towards good old white. It is just too difficult to stock colours.
Just ask around they do have some colours in some materials. olewill
We have coloured sails on our cat, white can be a bit hard on the eyes in the tropics, they are lite chocolate, much like the old traditional sails, mast head furler and fully battened loose foot main, Spinnaker is red and yellow, reacher is is a light tan.
All very easy on the eye, I did some racing on an S&S that had a black spinnaker and was very surprised at how well it could be seen at night.