Been noticing the last few years the used Castrol GTX 10-30 has a red tint to it at the 3K oil change. It will have a red tint on the dip stick at 1K miles. It comes out of the bottle a light amber, no sign of red. Whats going on here?
Different additives change color to different extents when exposed to heat. Red is a common color that forms but some much more so than others. I think this is the cause of the red oil you are seeing.
Base oils can turn brown solely from heat but they are rather well protected from that early in the oil's life by antioxidants, so this is more of a factor in motor oil that has taken a beating.
Used motor oils obviously also disperse combustion products which tend to make the oil look black.
Then there is water...oil/water emulsions tend to be yellow.