While this makes sense with some oils, with a great many oils it does not. The grapefruit, for example, has color like the rind from which it is pressed – pink grapefruit is pink, ruby red, red, etc. It is a ‘cold pressed’ essential oil, rather than a steam distillate.
If you did the same with any pure ruby red or pink grapefruit essential oil, you’d find the same. Further, these oils are never found adulterated, as their cost (relatively) is low, and the yield from the plant high.
And if you did the same with Blue Tansy or German Blue Chamomile…you’d get blue.
So, really, it’s only worthwhile for a few oils. Sandalwood, Rose Otto (not absolute), maybe Neroli…Helichrysum. These would be oils that one would want to test in this manner, as they are sometimes ‘extended’ with DPG, which would, in fact, leave an oily residue.
Hope this helps!