"Roy Krenkel was a key factor in the 1960s revival of my grandfather's writings. 
Krenkel's illustrations forever secured his position as 
one of the all-time great Edgar Rice Burroughs illustrators."
– Danton Burroughs

After World War II, Roy G. Krenkel took art classes from Burne Hogarth at the Cartoonists' and Illustrators' School. His contributions to the comics field remain limited, since he never created a comic on his own. He started out doing comic book art in the 1940s and 1950s, working for companies like ACG, Atlas, Eastern, the legendary EC, Harvey and Warren. In fact, Roy Krenkel specialized in drawing backgrounds. He assisted famous artists such as Frank Frazetta, Al Williamson, Wallace Wood, Joe Orlando and Gil Kane. His credits include Atlas and EC titles like 'Adventures into the Unknown', 'Forbidden World', 'Marvel Tales', 'Weird Fantasy' and so on. Krenkel eventually abandoned his comic activities and devoted himself to illustration, most notably cover illustrations for books by Edgar Rice Burroughs and Otis A. Kline.

The cover painting for JUNGLE TALES OF TARZAN is at ERB, Inc.  The original of A FIGHTING MAN OF MARS is in my own collection.  I furnished David Spurlock with a transparency of the painting for use in THE ART OF ROY G. KRENKEL and I don't know why the colors are so bright in the book, since the original painting is not nearly that bright.

The later Ace reprint of THE MAD KING (very blue in color) was printed from a quick recreation of the painting because, after borrowing the original painting from Ace, Frazetta didn't want to give it back (the bluish coloring is because of the photographic printing process).  The cover for the later Ace edition of SAVAGE PELLUCIDAR was originally a painting that Frazetta  did for the movie, LUANA.  Because of his tight deadline Frazetta felt he didn't have time to paint a new painting so he revised the movie poster painting taking out the black panther and replacing it with a sabretooth tiger.  Late in his life Frazetta did paint a recreation of his original Ace cover for THE LOST CONTINENT for himself.

The only Frazetta painting that some hack in the Ace Books office retouched is THE MONSTER MEN.  Don Wollheim was bothered by the cleavage on the girl and had the hack poorly cover it up with a dab of red paint.   The original still features the desecration!

Just because a particular scan or printing of a painting exhibits color differences, doesn't mean that they were repainted or revised.  It is merely due to the differences in scanning or printing methods.

I discussed Frazetta's Burroughs covers with him on several occasions and he answered all of my questions freely.

I hope that what I have written is of some help.  Use as much of what I have written as you wish -- all or some of it.