Uncirculated Natural colors range from Dark Brown to Red Orange. The grading companies use Brown> Red Brown > Brown
Coins can be "doctored" to be a multitude of colors from Blue to Purple. In natural toning I've noticed from red/brown to a golden orange. But I have one cent that is dark red in the front with purplish hues and blue in the reverse , is it natural I don't know but it nice.
Once a coin gets into circulation the only natural colors are brown as that is what copper tones to. Anything else is not natural.