I like to choose my nail colors based on what suits my skin color and what my mood is. I find that I cannot use very dark colors like black or navy blue, so when I feel sad, I go for light, neutral colors. When I feel happy, I choose bright colors.
When there is a special occasion, it is nice to be able to match your nail color to what you will be wearing. But since I tend to get lazy removing the color from my nails (especially if it is a dark color like red or very glittery), I end up choosing safer colors so it will match everything I will wear for the next few days. (e.g. red, pink, gold)