In the watercolor painting "Midsummer Morning" 1908, his color range is restricted to blues and yellow/orange colors. You could achieve this effect with just ultramarine and raw sienna, and maybe a raw umber for darks.
The warm-against-cool is orchestrated throughout the image as a whole, but also in its microcosm of small planes. In these details of the image, note how the far forest is held to an atmospheric light, cool value, with the nearer tree trunks edge lit and receiving warm reflected light.
In the shadow side of the sheep in the sunlight, the top planes receive blue skylight, while the bottom planes receive warm reflected light. The bellies of the sheep in shadow receive much less of that bouncing warm light, so they're darker.
Heysen himself said, "Keeping the trees solid in the morning light was the difficult thing, I think it was something I was striving for all my life really. The subtlety of the tree combined with the beauty; the bulk, the solidity of the tree, and the character of its growth. And the movement, that’s something we mustn’t forget … I had my special trees, and they altered their appearance—the time of the year and the angle of the sun made all the difference. You could paint a tree one day and get all its various facets. And the next day it would be a different tree."
More about Heysen at the website of the National Gallery of Australia