Rowan looks good in blue (all shades but especially brighter and/or lighter blues), dark or bright red, green (all shades), and sometimes orange although usually I like more contrast. I kind of want to try purple on him, but everyone already thinks he's a girl, so I haven't. Almost all of his things are blue so I guess that's "his" color.
Riff looks good in anything bright, really. Blue, green, red, orange, and although I'm not usually a fan I suspect he'd even look good in yellow. His leather collar is turquoise which looks great on him, and I want a wider ranger of colors for him.
I don't do black for either one of them. I just don't like how it looks on dogs that don't have black pigment.
I can hardly wait to have a female dog someday because there are SO MANY adorable girly collars. Plenty of cute masculine/neutral collars too, but oh man, I covet the pinks and the florals etc etc.