Comet colours would be extremely bland and boring to a human eye (and to the regular cameras we use on Earth) - the Sun is far, no atmosphere to have meaningful shades of grey, and the material they are made of is not really vibrant at all. The budget is better spent on instuments and the cameras that capture images in the invisible spectrum (which could be always false-colorized into colorful "photos" of galaxies and nebulae we are all used to see)