[Best way to adjust white background for headshots]

I'd like to adjust the white background in a series of head-shots to be white with RGB values of 234.  I first choose the Color Picker Tool, hold Alt and left click to pick the current color of white of the background.  Then I choose "Select Sampled Color" from the "Select" menu at the top of the screen, then adjust the the tolerance to best select the background.

Perhaps this is not the best method to select all the all the background shining through peoples hair.  But I expect that somehow creating a layer (that can be edited) that identifies the background pixels is step one.  Then step two,  change those pixels to the desired color.  Unfortunately Recolor doesn't have an RGB option.

I'm quite new to Affinity Photo and am hoping someone has figured this one out before