I like to layer because the glazing creates color showing through or strongly affecting the second layer. When areas are blocked out the first color is preserved. The visual affect is not the same as mixing the same two color together, the added colors play back and forth as transparent or denser overlays.
Working this way leaves a lot to chance, allowing the image to direct its next addition, block out or changes. Each additional color layer creates its own personality, mood and suggestions.
Often the image can be shown in all four rotations, each with its own suggestions, balance and impact..