Colours - Red

The complete opposite of the colour green is the colour red. While green was calming, safe, and inviting red is a big warning and alerts danger. We are naturally tuned to be alerted by the colour red, the best example of this is blood. It takes a human, (and animals in general) no time at all to learn that blood = stop and pay attention. Stop signs, and emergency vehicles are red for this reason. It's hard not to pay attention to the colour. You'll notice McDonald's. Wendy's, and, Burger King all use red in their logo. This is about as close as you can get to yelling, "hey stop here" on a sign. Red is used the exact same way in video games.

Blood

Blood in video games has become a standard since Mortal Kombat (there was blood before but it was the first game to popularize it). Not only does blood give a sense of realism but also helps direct the players attention toward the action. The red blood stands out and it is easier to aim at and see exactly where your eneimes are. This is why blood isn't just added to a game it is also exagerated. Look at how blood draws your attention in these examples.

Mortal Kombat

(Rather then just seeing the fighting you can also see the impact of your hits. When this game came out in the arcades it was the first fighting game to use blood. Not only did it draw attention of the players but crowds would normally form around the cabinet.)

Gears of War

(a game where you cannot just run-and-gun. You need to find cover and react in a strategic manner. Everytime you hit the enemies a great deal of blood come out. This lets the player know where their targets are. Also the enemies and the backgrounds have the same colour scheme (grey eniemes and grey levels) so the blood adds much needed colour to the game.

Uncharted

(I have a Japanese PS3 and when this game first came out the blood was censored on all Japanese PS3s. They have since fixed the error. I played the game a bit before patching the game and noticed it was hard to tell when I was hitting my target. With blood the game became easier.)

Mad World

(The game only has 2 colours red (blood), and yellow (for sound effects). The rest of the game is black and white. The blood red really stands out against the stark black and white game. This further exagerates the blood by allowing red be the only real colour on the screen. It also makes the action much more exciting.

Hard Levels

Just like how green is a good colour for the easier levels, red is a great colour for the harder levels. A lot of games have had lava or fire themed levels and they are usually the harder levels in the game and have a vibrant red colour scheme. However, red levels don't always need to use lava or fire.

Oblivion

(in contrast to the peacful overworld the Oblivion planes are filled with harder battles).

Super Mario 64

(One of the levels that has you doing the most tricky platforming is a lava level)

Mega Man 2

(Heat-man may not be the hardest boss but his level required skill).

Bowser's Castle in Mario Kart

(some of the harder courses in Mario Kart are red).

Attention

Finally red can be used to just grab your attention without using blood. Some games have just used the colour red because it does a great job of standing out from most other colours.

Gears of War

(When you are getting close to dieing, a red Gears of War symbol will be flashed up on the screen. Warning you to take cover).

Mario

(You think it is a coincedence that the most recognizable character wears red clothes? Its a great way to make Mario stand out in all the colourful levels he is in.)

Health bars

(most health bars in games are coloured red. The health bar is important for the gamer to notice so having it lets it stand out even when not in direct line of site. Also having the bar red sybolizes blood, since as you lose health you are losing blood even if the blood is not shown on the screen.)

So while green was the safe and clam colour, red is the danger and warning color. It can be used to either catch the players attention against everything else happening on the screen, or it can be used to warn the player of danger.