At an engine assembly plant in the UK is was told that painting the inside of the pan happened because they painted the outside. They are dipped and it helps to guarantee the engine is assembled "clean". An unpainted pan it was said is more likely to introduce small particles into the engine. I was told that being clean is really important. If you got caught touching the parts, fixtures or tools without gloves you were gone. Same with keeping "booties" over your shoes or walking into or outside your work area without changing.