I am not sure I understand what you are proposing - the jambs/channels where sashes go up and down should not be painted at all, or anywhere it could interfere with operation of the window and sashes moving freely. Actually I would just leave all window parts unpainted - sashes, jamb and trim around window as is. Wood looks to be in good condition, trim is not wide or worth emphasizing, plus why make busy work for yourself unnecessarily? If there is painted trim elsewhere, that's ok. Just keep everything consistent throughout the house- e.g. all base boards painted, but trim around windows not painted.

It seems your jamb extensions are already primed or painted , i would paint them. If you are talking about the little bit of the frame which is stained I would leave it,

@toddinmn Yes, the jamb extensions will be painted. The windows are vertical geain fir, and I can’t decide if the bit of jamb the extensiok extends from should be painted as well, leaving only the frame and sash natural. They are new unfinished windows and it’s being professionally painted so difficulty isn’t really a considerarion. I know on exterior windows, if the trim is to contrast, the frame should match the trim and only the sash be a different color.

@Delete This Everything is new, so ”ad-is” would be pre-primed jamb extension and casing and unfinished vertical grain fir window :). The whole house has painted trim and windows. There is natural fir cabientry extending into the dining room from the kitchen, so we decided to put fir windows in the dining room to bring that element to the other walls. I just think it looks a but odd to butt the painted jamb extensions against the unpainted window jamb instead of changing surface finish at the change in plane.