Why is the sky sometimes gray?

The sky looks gray when clouds fill it with countless water drops or ice bits that spread sunlight in many directions. Instead of mostly blue reaching your eyes, the colors get mixed together, and a thick cloud layer lets through less light, so the world looks muted.

One lovely clue is the softness of a cloudy day. Shadows fade because light is arriving from the whole sky, not from one bright spot. Fog is the same idea close up: a cloud around your ankles, nose, and streetlights.