Just to show what I mean (and because I'm procrastinating from RL), I took 2 pictures of Gretel on the left with twinkle shade, and Hansel on the right with natural shade.
This first photo is taken in Cal Windlight which is a strong even bright light often used for fashion photography which brings out the shades. Here it is easy to see that the twinkle has a greenish cast to it and the natural is warmer and cleaner.
This second photo is default SL lighting taken at noon. In it, Gretel appears to be brighter because of the twinkle shade, and Hansel, being natural, has nothing that lights him up.
Quick edit to add that I always turn my glow down from the default 2 to 1. I find the default 2 to be so abominable that I cannot cope in SL with it that high and some of the kitty shades are overwhelming at default 2.