This setting rifts off the strategy game
Sorceror. Basically, six high magic universes are in contact with a low Magic universe. Each universe has its own Manna. Each different manna empowers a different magic style/system. Humans perceive these magics as having color and they magics seem to work somewhat like colors on a color wheel.
The standard GURPS Magic system, for which this whole world is a Low Manna setting, is called
The Grey Art. It works the same way everywhere in this setting.
Each of the Color magics, which are named for different gems (Ruby, Opal[FireOpals], Topaz, Emerald, Saphire, Amethyst) are high manna in their dominant areas or in white areas. Color magics are weaker in areas that aren't either White or Dominant their color. Example: Red Magic has High Manna in Red areas, Normal Manna in Violet or Orange areas, Low Manna in Blue or Yellow areas, and No Manna in Green areas. All the other color magics follow this pattern. Color magics function as low manna in "Grey Areas."
Grey areas are the normal state of this realm/universe when other realities don't impinge on it. White areas are where all six other areas equally intrude on this world. Twenty percent of this realm is "Grey." Two percent of this realm is "White." The six colors have 13% of this realm each.
More later.