Objective: Your goal in mono is to change the color of the screen from black to white or from white to black(there's a player controlled setting to alternate between the two). In order to accomplish this task you have to shoot your enemies (the colored circular blobs). There are three types: red, green, and blue. Each type leaves a splat of its color and when all three are combined in an area of the screen, the area changes to either black or white(depending on your setting). The process of doing this creates a very colorful playing field and changes the behavior of your enemies. Your score is represented by the percent of the screen you have colored and the game keeps a screenshot of the high score. This is an unusual and interesting concept but doesn't really do much for the game. Unless you plan on shooting your enemies in specific locations, you're probably just going to run around and kill everything in sight as soon as possible.
Obstacles: Enemies that run over red background will shoot at you, enemies that run over green will speed up, and enemies that run over blue will try to home in on you. These are the only things that result in different enemy behaviors, it's not a very complicated system and the game would certainly be more interesting if there were additional behaviors or types. It isn't really a critical issue with the game design though. If you collide with an enemy or get hit by a shot from one, you will lose energy and if you run out of energy you die. My first gripe with the game is that there's no explicit indicator for how much energy is remaining so you can't see the effects of energy giving power ups. Instead, the screen wobbles faster and faster the more damage you take and your avatar will start flashing if you're really low on energy. This leaves you with a very qualitative estimate of how hurt you are. It's also rather annoying because at high damages, the game becomes much harder to play.
Goodies: There are four kinds of power ups. The first one I've already mentioned; circles give you extra energy but you can't really see that they're doing it. The second power up is a star. Stars upgrade your weapon, usually... The problem with stars is that the upgrades aren't really balanced so it's always an improvement to grab one so sometimes they're more of a power down than a power up. This is the second big flaw in the gameplay because if you don't take the power up, you'll be penalized(the power up explodes and removes some of the color). You might be inclined to just view the penalty as an interesting trade off for keeping your weapon but I just find it annoying. The third power up looks like a plus sign, it causes an explosion of shots that will pop enemies in all directions. It isn't nearly as good as it sounds because there aren't that many shots in the explosion. The fourth is a swirling orb that gives you limited protection from harm. Again, because you have no explicit display for the amount of energy that is left to you, it is difficult to understand the exact effects of this power up. I think these power ups could have been implemented much more effectively.
Sound: Some people will probably find the music obnoxious. I find the music to be tolerable background noise. It doesn't really matter because you can set the music to whatever you like by swapping out the mp3 in the folder and you can easily toggle it on or off. The sounds of shots aren't particularly obnoxious so you may as well leave them on. Overall, the sound is like the rest of the game, it does its job but it isn't really outstanding.
Recommended Settings: The default controls are probably the best. You move with the mouse and control firing direction with the wasd keys. If you want to be careful about where you kill enemies, you should turn autofire off but when I tried this I did worse. The graphics settings should be turned all the way down for two reasons. The first reason is to get a better framerate and the second reason is simply for visibility(it is very hard to see what is happening with high graphics settings).
Conclusion: Despite a number of flaws, Mono is a fairly fun and fast paced game. It's a good way to kill a few minutes here and there and I'd recommend a download simply because everyone needs a game they can kill a few minutes here or there with.