Great question charli:  French can be great on the fingers but I like when the fingers and toes match and french on the toes is almost never good (If your toes nails are long enough to french them, then they almost assuredly too long for my liking).  If can work sometimes but it is fairly rare... but that is IMHO.  If you do choose to french the fingers, then white, black or standard red is best for the feet.

regarding color, if a woman has lighter skin, then black/super dark reds look hottest but if she has fairly dark skin or is very tanned, then a hot pink or white looks amazing.

Wild colors like greens, blues, yellow can often be distracting and can seem slightly odd or unauthentic on the wrong girl (like she is trying too hard).

Of course, its not like I have an opinion, a foot feetish or think about this topic at all  ...so what do I know?