Red for me, particularly on B&M track. I've always regarded Dragon Khan to be perhaps the most aesthetically pleasing coaster in existence. It's just a shame they chose to place Shambhala behind it dwarfing what was once an incredibly imposing coaster. As much as I love Shambhala, they could have easily built it elsewhere in the park.
Aqua/teal/turquoise type colours also look very good.
As for colour schemes I don't like, there's nothing worse looking than a white tracked steel coaster with white supports which is oddly enough one of the most popular colour schemes in Japan.
Some white-tracked coasters can look very crisp and modern for their first few years of operation (see Time Machine and Shambhala) but when rust sets in and the track becomes dirty, they just look awful (see Pyrenees at Parque Espagne).
I also don't like Skyrush's colour scheme at all. Some yellow-tracked coasters can look great such as a few examples of the Batman clones but for whatever reason, the powder blue and yellow combo offends me.