Blue is the colour of the sky. Well, when the sun's not setting, and only during the daytime, and not on Mars or other planets that have more pinkish atmospheres.

Pink is the colour of the sky on Mars. It is also not the colour of the skin, because skin comes in many colours but it does not come in pink except in certain areas.

Yellow is the colour of cheese. Cheese is delicious, except when it isn't and when it isn't it's often quite horrible. Yellow is a good colour whatever, however, and is also almost the colour of eggshells.

Green is the colour of photosynthesis, which makes me think of grass and trees and other plant-like things. Grass is excellent for walking on if it isn't too deep, and even if it is a bit deep you can usually wade your way through.