Two schools of thought on this issue.
One says color sand within a day.
Other says wait several weeks to color sand.
Later method says that if you color sand early and buff after several weeks the surface will not look as smooth. The theory is the clear is still settling within that first few weeks and if you wait to color sand you will get a much smoother surface that will hold it's smoothness.
Best method IMO is to color sand several weeks after the final clear. Color sand the entire vehicle and place it in the sun for several hours before you buff.