When you have a lot of bright highlights in a painting that otherwise has subtle, moody greys, it can be tempting to pick out one or more of the bright colours for the frame. In this case, that would definitely not work.
The overall mood of the painting and what the artist is conveying have to be respected and not opposed.
There is joy in the brightly coloured flowers and heathers in this heathland landscape. All of the rest of the scene is painted in delicate, misty tones with a very limited palette, allowing the bright colours to “sing”.
By echoing these muted tones and not dominating the overall picture, the frame works with the image and supports it. The frame becomes a window through which we see and understand the beauty in the piece, and indeed get the feeling we can step right into it.