It is not straightforward – first, you need to choose a custom colour via Wallpaper settings menu Colours section, by clicking on "+" button and choosing any custom colour, THEN, at the top section of this settings menu, a "Custom Colour" box appears with your chosen colour. IF you close the settings, and open them again, clicking on this custom colour will open the bugged popup, where dragging the cursor around is certainly taking more frames that it should, resulting in it lagging crazy