How do you fade one color to another with oil paint without getting the mixed color?
There are green areas in the photo, although they are not bold.
Maybe your solution would be not to fade but to juxtapose bold colors.

Yet, if you want to fade, painting by layers will have a cleaner mix than painting in impasto (glaze a very thin blue over the yellow). Also you could use dots of each color next to each other for a vibrant optical blending (like impressionism or pointillism).

Otherwise, first fade the colors toward grey before fading.
One of the ways to desaturate the colors is going into non complementary biases, like fade from a blue-green (cyan, phtalo, Prussian), into a blue-red (ultramarine, cobalt), fade that to a yellow-red (azo, cadmium) then a yellow-green (lemon yellow, Aureolin). The intermediary color will be greenish but muted.

Another way is to use ochres and earth tones (transitioning with a Venetian or yellow ochre, or muting the green color with green-earth or another greenish brown paint), and yet another way is to use opposite colors to mute the intermediary color (i.e. mute the green with a glaze of red).


However, I think the best way to achieve a clean contrast with high colors is to mix as little as possible.