The Subtractive Primary Colors are Cyan (sometimes called blue), Magenta (sometimes called red), and Yellow. They are used in printing, painting, and filtering. Cyan pigment reflects blue and green (filters out red). Magenta reflects red and blue (filters out green). Yellow reflects red and green (filters out blue).
So Cyan plus Magenta should reflect (transmit) only blue. A combination of the three should leave only black. alas it does not do so in practice, producing instead what can be described as a muddy brown. That is why color pickers are usually RGB or CTMK, not RGB or CYM. "K" is for Black = don't ask.
This game uses the idealized CYM model, so:
PrimaryColors I, used the additive colored lights ( Red, Green, Blue) to turn a black stage white. This game, PrimaryColors II, uses the subtractive primary colors (Cyan, Magenta, Yellow), as pigments, to turn a white canvas black.
The point of the game is to fill the square outlined in red with Black Squares.A combination of the three subtractive colors will produce a black square (+). The black square stays there, cannot be changed or overlaid. Incompatible colors such as Cyan and Red will produce a Gray Square (-), which is also permanent. Black adds one point and Gray subtracts one point. All Black gives the maximum possible score.
The left and right keys move the falling square left and right. While it is being moved it does not move. If you move it off the edge it will appear at the other edge, possibly with a different color. Similarly the up down keys move the the horizontally moving square up and down with the same restrictions.