What are some references to the colors gray, red, and yellow in the book The Grapes of Wrath?

From the text:

HIGHWAY 66 IS THE main migrant road. 66—the long concrete path across the country, waving gently up and down on the map, from the Mississippi to Bakersfield— over the red lands and the gray lands, twisting up into the mountains, crossing the Divide and down into the bright and terrible desert, and across the desert to the mountains again, and into the rich California valleys.

And on the level vegetable lands are the mile-long rows of pale green lettuce and the spindly little cauliflowers, the gray-green unearthly artichoke plants.

TO THE RED COUNTRY and part of the gray country of Oklahoma, the last rains came gently, and they did not cut the scarred earth.

Joad took a few steps, and the flourlike dust spurted up in front of his new yellow shoes, and the yellowness was disappearing under gray dust.