Sometimes you'll have nothing but a white beam of light, and you'll need to create blue, red or green lights to solve the puzzle. Feed the white beam into a splitter block and the new colors will emerge from the other sides of the block.
If a beam of light won't pass through a gap in the wall, that means there's a color crystal in the way. The color of the beam must match the color of the crystal in order for it to pass through unimpeded. Use mirrors to divert beams around each other and squeeze them through the correct crack!
If you're really stumped and can't see a way through the puzzle, you can always hit the question mark icon in the top right-hand corner of the screen for a full solution via the Vision Crystal. It's a bit of a spoiler though, so try to avoid using this unless you really need it.