Fixed bug: color plates thinking they are walls

Symptoms: Stepped on a color plate and player turned black on level 3.
Fix: Apparently the color plates had wall sprites for some reason, so
they didn't really have a color on their own.
Prevent combining when having black

Can no longer press Z to combine when either child color is black or
combining would result in black color.
Removed all plot stuffs
Disable combining color on earlier stages

Early stages (1-3) have not been introduced color combining yet, so
accidentally combining color would be confusing.