Why are the trousers of the USMC dress blue uniform sky blue and not the same blue as the jacket?

Because of tradition. When the Marines we aboard the ship they would take off their jacket, and being on the deck all the time caused their pants to fade, but their jacket didn't.

Actually. The reason the coat is a darker color is because they would wade into swamps during marches, then out, then back in. The water only came up to about their waist, so it would fade much quicker than the jacket which remained out of the water.