[NTG-context] Feature Request: define colour in relation to existing colour

Hi, What would it take to extend \definecolor so that: \definecolor[ColourA][ColourB][t=0.5, a=1] defines a new colour (ColourB) based on an existing colour (ColourA)? I know that \definespotcolor[ColourA][ColourB][t=0.5, a=1] works, but it seems like \definecolor would also be a natural fit. Kind regards.