faint orange marks have happened to me when the water temp was not hot enough or the soap not strong enough to dissolve grease on the clothes,
like butter someone wiped on their lap after a piece of toast,
it is from iron in the water.
That goes the same for radiators,
they are made out of iron and even though you may not see them being rusty they leave a stain on wet clothes from the iron leaching through a chip in the paint.
There is a product my mother has used called "iron out" I believe,
she says it worked great.