Hey! I don't think this is possible with how the name colour tag works on CubeCraft. For that, an entire new system should be added to include all rgb colours, which requires a lot of implementing of different add-ons i believe.
How the colours work right now is with the default minecraft colour codes, using the following codes you can find in the image. These sadly do not include magenta, hence it is also not available in /colour
Hope that gives a bit of background for you!