Good explanation in AKC-Poodle-Standard
An excerpt from the AKC-Poodle-Breed-Standard:
... Color
The coat is an even and solid color at the skin. In blues, grays, silvers, browns, cafe-au-laits, apricots and creams the coat may show varying shades of the same color. This is frequently present in the somewhat darker feathering of the ears and in the tipping of the ruff. While clear colors are definitely preferred, such natural variation in the shading of the coat is not to be considered a fault. ...