It would be really nice to be able to see a live preview when selecting a sampled colour in Pixel Persona as an overlay or black and white mask. Currently it shows an approximate result while dragging a slider ( dashed lines ). Extracting clour range ( sampled colour ) and using it as a layer mask or fx layer is a common practice in compositing. Architectural visualizations for example. I can select a sampled colour and click Refine to check if what i did was ok, but it is always a guess. Designer is a GREAT program, but I really miss this feature. I am testing Affinity Photo Beta for Windows and I can't see this kind of tool as well. ( Pixelmator has it BTW ;D ). I know it is a beta so maybe it is a good time to point it out.
Performance-wise, maybe taking a screenshot and previewing selection mask on a low-res image is a good idea?