Just wanted to say, great little game Housemarque worked up. A platformer that takes a great deal of inspiration from bullet hell shooters, specifically "Ikaruga" and puts it into an exploration based platformer (similar to, but more linear than, Super Metroid)
Gameplay in a nutshell: Bullets and enemies come in red and blue, so do you. By swapping colors you can become immune to bullets of your color and gain the ability to hurt enemies of the opposite.
As you progress you are given mobility and combat upgrades, often required to gain access to the next area.
The game features 5 areas and clocks in at around 4-5 hours total. It features campaign co-op as well as an Arcade mode meant for compulsory speed-running (you're given a small winddow of time in which to get from the start of a chapter all the way to the death of the boss. Requiring rapid precision color swaps as well as an intimate understanding of how everything patterns and the best time-saving path from A to B to C to D.
A good example of the Light/Dark gameplay mechanic can be seen around 1:45 in the video.
Optionally, If you don't happen to like narrated videos. May I recommend muting the above vid and playing the following Jan Hammer track in the background: