White and black are easy. However, maroon and a true purple are difficult.
I think the secret to painting white and black are to not use white and black for your base colors. For Black, I start with a dark blue, brown, green, purple, etc. This allows you to shade with a "true" black. Same for the highlights, use anything other then white and you will get numerous variations of black. Often times, I will paint on my base black and let it dry. Then add shadows and highlights. It is much easier to keep the paint looking "black" this way.
The same concept applies to white. Start with something other then a "pure" white. I use Tit white mixed with a dab of another color: Mars Yellow, Raw Umber, Burnt Umber, MArs Red/Chrome Oxide Green, Sepia, etc. The mix won't look "white" on your palette but it will on the figure. Just add more of this tint color for the shadows. For the highlights, I use pure Tit. White blended in wet in wet. For the most extreme highlights, paint on Tit White after everything else is dry.
I think the secret to painting white and black are to not use white and black for your base colors. For Black, I start with a dark blue, brown, green, purple, etc. This allows you to shade with a "true" black. Same for the highlights, use anything other then white and you will get numerous variations of black. Often times, I will paint on my base black and let it dry. Then add shadows and highlights. It is much easier to keep the paint looking "black" this way.
The same concept applies to white. Start with something other then a "pure" white. I use Tit white mixed with a dab of another color: Mars Yellow, Raw Umber, Burnt Umber, MArs Red/Chrome Oxide Green, Sepia, etc. The mix won't look "white" on your palette but it will on the figure. Just add more of this tint color for the shadows. For the highlights, I use pure Tit. White blended in wet in wet. For the most extreme highlights, paint on Tit White after everything else is dry.