Question: In my screen widget I want the register values have custom colour. I set a color for them, but still they are black/white colours (depending on theme)
The register values in the WebHMI are colourized using this rule - it is either default value color, or the color defined in the register STATE. There is a default register value style which overrides text widget styles for values. So, to make your register value of a desired colour, set a STATE for the register with the colour you want.