Gray is neutral too, but there are many tints of gray. Add a touch of blue for a cool gray, a touch of red for warm gray, etc...
Here's a wonderful painting of trees made with subtle neutral colors:
OK, it's almost finished.
If this had been painted in the 80s, it would have been a lot harder to read. Each major object (Rocks,Wall, Lockers, Furs, Benches) would have been painted harsh and clashing primary and secondary colors.
Colors straight out of the Cartoon Color cel paint tubes. - like the paintings I posted a couple days ago.
This BG uses more control to guide the eye and focus the viewer on what is important.
It uses neutral colors that subtly change hue, value and saturation -to keep it from looking dull and monochromatic.
The biggest contrasts in value and saturation are on the benches and this helps frame the bear furs and draw our attention to them.
The painting technique is very skilled, sensitive, organic and appealing too. I think it's Richard Daskas. The layout was drawn by Aaron Springer.

He has a slick style and puts just enough detail into the backgrounds and characters to give you a different experience than what you get from watching the cartoons.
The cartoons have the advantage of sound and motion but are limited in how the characters can be rendered, so good Golden Books like McGary's give you extra treats in the rendering.
I like when he does the 2 color limited palette pages too.
I always wondered what the pay rates were for each job. Does the layout artist get as much as the painter? Even though the painter does the most work?
This Huck and Friends book has everything an HB fan could want- a bunch of favorite characters in a rocket.
And such a beautifully rendered rocket too! I used to fake sick, skip school and stare at this picture for hours and wonder how many years of intense schooling, whippings and study it would take to become a giant of the Golden Book world of painters.
Sometimes McGary would use more grays and neutral colors than most cartoon painters (who tend to use too much primary and secondary colors.)
He used darker colors than the typical Golden Book.
I sometimes confuse his work with another good GB painter - whose name escapes me right now. (I think he did the Beany and Cecil Book)

I also like the way he painted all the humans' noses red, like they went around drunk everywhere.
Aww, how cute...

I also wonder why CG films have settled on making once cute characters as ugly as possible, when there is so much reference on how to render cartoon characters as appealing as their original designs and cartoons.
Like is there a law that says if you revive a classic character, you have to make him look uglier than real life?