Best way to adjust white background for headshots

[Rich DX]

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

 

[firstdefence]

What about after creating the selection you add a Fill Layer, this can be colour controlled from the Colour Panel and so you get to input RGB numbers.

Layers > Fill Layer

[NotMyFault]

Your question relates to one of the hardest and most frequently asked topic. 
 

If you have portraits with hair in front of background, it becomes hard to get a clear selection or mask, and both background and hair may have color contamination from the other side. If you only change a non-perfect selection of the background, you may get harsh color contrast if not removing color contamination.

Which method works best really depends on the actual image. If you can upload the image, or a relevant excerpt of it, we can check.

Affinity allows automatic color decontamination if using refine, and new layer output option. 
Other methods are using defringe filter, HSL de-saturation etc.

[RichardMH]

There's quite a few youtube videos on selection. Selection brush tool and refine selection can be good. Or with the uniformly coloured background, Flood Select Tool should work well. Control J then creates a layer with just the selection (so depending on what you've selected you might need to invert the selection) . Don't use recolour. Try adding a fill layer beneath the selected  image layer and open RGB in the colour tab. There's a hex option there too. A quick application to one of my birds. Info panel lets you check colours.