To answer your question, it is currently not possible to change track colour. The colour of tracks "ages" from bright red to purple for older tracks - I really like that.
Recently someone mentioned a Maxsea
feature "track colour variation by speed". That also sounds very useful for intuitive analysis at a glance. Perhaps the two could be combined by having older tracks fade into tones of grey or darker colours, whilst having boat speed expressed by colour variation. Sounds horribly complicated to implement though.