If I'm in doubt about a painting I'm working on, as with this Fool dilemma, I just put the image aside for a couple of weeks and don't think about it. Then the answer will hit me.
Yes, the white rose stands out better with the darkly silhouetted Fool, which is a kick-ass composition. The Fool-dog-cliff trio is a strong compositional and card meaning element, and I personally would prefer to see them coloured the same as each other to maintain that compositional strength. Though, white (or cream) colouring does suit their carefree meaning. What would you think of a deeply red to red-orange sky, which only becomes yellow at the sun's disk by the white Fool's head? What about a dark (red?) outline for the Fool composition, which would help pick out the rose? (That would mean tweaking the trees' colour, so the red/green contrast wouldn't look angry; but there are, for example, blue spruce, golden spruce, autumn-golden larch/tamarack....at least the trees are "just" supporting players.) Oy, the more I think about it, the more I like the dark Fool. It ain't broke.
Perhaps we're all overthinking the matter--something the Fool would never do!