>>7660326It's not really red tho, just a warm earth tone.
When you start painting on a white surface, all your colours and paints look dark, it's harder to get the values right because your canvas is already the lightest value.
As for the colour, it really depends on taste, some people paint portraits on a green first layer, some prefer a warm undertone. Sometimes it helps balance out the painting, if you know you will use a lot of blues, a warm orange first layer neutralizes that blue a bit which can be a desired result.
It all depends on what you want.