It depends on the person and what it is going to go on but I like the military colors for most of my knives.
On neutral color knives like Sebenzas colors like yellow and blue (Nautical look)
orange-black or purple-black look good.Purple-black looks really good.
On tactical knives like Striders and the like almost any of the military colors go well.
Heres some of the combos I like best and yes the body style does make a difference in how the colors look.
Navy-yellow square
black-yellow square
black-toxic green square
ACU--desert tan or foliage green round or square
Woodland-hunter green round
OD-marroon round
Not saying that some of these do not look good in round and square but just what looks better to me.
IMO the red-purple would look better round.
To tell the truth I have not tied a color combo yet that I thought looked really bad,even a coyote brown and yellow that I thought would look bad,turns out those two colors together are in style right now
Well just thought of the color that I have tied together that I do not like myself but thats just me and they are yellow and toxic green,to lemon-lime looking for me.