The RGB mixer is not happening. Half the people would use colors that screw up contrast or text legibility in apps. Some colors would by chance work in some apps, but not in others. Of course people would end up blaming app developers or the OS. WP doesn't need that. As it is now, app developers have a specified set of very saturated colors to test against, not an endless range of colors users may or may not use.
We might get more color choices though. What I'd also like is the ability to override colors for individual tiles. IMHO the OS should force every app to provide three choices: default, custom color (any one of the standard accent colors), or transparent.