Hello! Sorry about it, this is my first game and know it is a bit confusing. Basically, you can use LMB to shoot a color, and if you shoot towards another color with it, it will try to mix the two of them. The possibility of the mix happening depends of the room, in each room you can see which colors are the primary (they can't be acquired mixing other colors), and which are secondary(by mixing two of the primary), this is based in the color models CMY, RGB and RYB.
So for example, if you are in the RGB room, if you see a red circle, and you have a green bullet, and you shoot the green to the red, it will result in the mixing of both, giving the yellow secondary color.
The middle mouse button basically fires a shot that will replace your current bullet color with the one you shot.
The substractive bullet (RMB) is explained in the description. Basically, if, like in the previous example, you have yellow (mixing red and green), and you have a green bullet, if you shoot the green bullet with substractive mode, it will try to take away the green color from the color you are shooting at, so if you take away green to yellow, it will give you red, because it is the other primary color necessary to make yellow.
You can use substractive bullets in triangles and additive in circles, and when you substract a color from a triangle, it will send the result color to a predefined circle (isntead of giving you the color).
The circles receiving the result of the triangle can't be used to mix colors (my fault), my intention was that they could be used to mix colors too like the other circles.
To advance, you have to shoot each door with the color they are made of, and then go through them.
Hope it helped a bit and thanks for trying it out!.