I'm looking for the source or information about an ancient watercolor pigment called, Rimington's Sky.

Answer 1

First, I doubt if any watercolor pigment named 'Rimington's Sky' would be very ancient. The early artists simply didn't give pigments fancy titles. And in the earlier days painting (the name is English) such paint pigments mostly were created locally and the colors varied from area to area and from country to country. Standardized colors with standard names didn't come about until the mid eighteen hundreds and later.

If there is such a watercolor color on the market today by that name most of your larger art supply stores should carry it. Try Dick Blick or Pearl. They both have on-line stores and web sires.