First thing I should note is that I'm scanning these panels out of a 33-year old newsprint comic, one that was alraedy a reprint of the original Galactus story, so there was likely some resolution lost and who knows what else before 33 years and my cheap scanner got to it.

That being said...
Kirby didn't do the coloring, but even if he had, the results would have been similar.
Have they taught you guys what it was like to hand-cut color separations?
It was a far more restrictive palette a colorist had to work from in 1965, and believe me: The job was a tremendously painstaking pain in the posterior.
I watched someone hand-cutting color separations for a cover I had done in 1986 and knew immediately that job was the last one I'd ever want to have.
And personally, I'd say the coloring was fine for the style of artwork and tone of the story it served, definitely by the standards of 1965.
And I've seen an otherwise good colorist completely overwhelm material that called for a more muted approach.