For example, the colors used on this page: http://reference.wolfram.com/language/ref/ContourStyle.html
or here: https://www.wolfram.com/mathematica/new-in-10/plot-themes/default.html
or here: https://www.wolfram.com/mathematica/new-in-10/enhanced-visualization/new-default-styles.html
(Look at the density plots or contour plots.)
An example:
I searched online, but can't seem to find any documentation or discussion of it. I searched the color schemes (http://reference.wolfram.com/language/guide/ColorSchemes.html) as well, still can't identifity which one it is.
Thanks!