This is what wiki had to say:
The term derives from the use of "red" as a color metaphor for race following European colonization of the Western Hemisphere, although initial explorers and later Anglo-Americans termed Native Americans light-skinned, brown, tawny, or russet. According to historian Alden T. Vaughan, "Not until the middle of the eighteenth century did most Anglo-Americans view Indians as significantly different in color from themselves, and not until the nineteenth century did red become the universally accepted color label for American Indians."
Doesn't mention anything about scalping!!!