Mix a good gray for your aspen color with hardly any white to lighten it and add some of the background color to the mixture (in this case, dark greens or purples).
Lift out the aspens from a wet, already painted background so that they automatically have some of the background paint influencing them. Then you can lightly paint over what you’ve lifted out.
Paint them over the background, darkening them if needed (less white in the mixture) then soften the edges of them by bringing their color out into the background a little then painting the background color back in toward the edge of the aspens.
I hope you’ve enjoyed this post, and don’t forget to click on the image(s) to enlarge them. If you’d like to see more aspen paintings in watercolor and oil, please visit my web site. Look under Landscapes and also Plein Air Studies. Thanks for being an aspen lover and for reading my blog!