About 5-10% of males have some form of colorblindness. What's the best way to ensure you are not turning away 5-10% of your male population from your game?
Just to note colorblindness does not mean people see in black and white. It usually means they see less green and get red/green confused easily. Colorblindness comes in degrees. Some forms are mild some are extreme.
Don't make game critical functions or gameplay based off of the colors that might confuse players.
Let's consider a game like Bejeweled. Let's say that a color blind person can not determine the difference in the red or green gems. They are going to have a lot trouble swapping gems if they can't determine what color a gem is.
You could fix a problem like this by including a second way to differentiate between the red and green gems. One example would be to give them different shapes, such that each color has its own unique shape.
Use things other than colours to different key games items.
Patterns and Shapes are very helpful.
Also this Vischeck website provides very good information, and a set of photoshop filters you can use to check your assest
The most basic answer is to use shapes, but that doesn't solve all the problems. What about colored text hovering over player's head to show their game name?
Here are all the fixes for colorblind issues I know of:
This is an interesting topic, I often refer back to this page: http://wearecolorblind.com/articles/quick-tips/ and the resources linked above seem quite good too.
Some of the issues are not as severe as you might think: for example red and green gems in a matching game may be perfectly fine if one is brighter (usually that would be green).
I actually had the chance to test Audiosurf on a colourblind user for a class, and found there was only one issue. For those who don't know it, Audiosurf involves matching coloured blocks (red, yellow, green, blue and purple) in most modes, however the "mono" mode reduces them to coloured blocks that change colour and grey blocks. In mono mode the objective is to collect all the coloured blocks and avoid all grey ones, but at some points they looked identical to the colour blind user.
Audiosurf now includes the ability to change block colour settings. Valve games also often have a "colour blind" mode which may be worth looking at. So I'd say with so many colour blind people around, you may be able to find some test subjects of your own and check any gaps with the vischeck plugin linked above.
I'm mildly colour blind and always find that I get along best with games that use very contrasting colours. The best example I can think of is TF2 where everything was either red or blue with very little exceptions. You would have to be severely colour blind to have trouble with two very different colours like that.
My father has some mild color blindness, and my cousin has a severe red/green blindness (he see both as yellow... something that makes things worse, since when he see a colour it may be yellow, or red, or green...)
My family feared that I would be colour blind too (I DO confuse some colours, mostly blue and black and dark blue with dark green... but it is not colour blindness it seems), so we researched a lot...
Basically, 7% of the US population (seemly worldwide is 10% among caucasians and maybe asians... I have no data for black people) is colour blind in varying degrees but of the same type of my cousin (ie: see both red and green as yellow)
So, the solution is basically don't use red, green and yellow as important gameplay elements...
I for example made my current game based on colors, but you can still understand the game if you only see the difference between red and blue, and the majority of the population can see the difference between those two colours (they are on the opposite sides of the spectrum and have mostly no overlap in our "sensors", even if the person have only a single colour sensor in his retina, he will see one of these two colours and see the other as yellow or gray, but unless you have some bizarre brain problem, it is impossible to see red as blue, or blue as red)
A BAAAAD example is Bioshock 2, it has a minigame where the background is yellow, and you have to stop a pointer above either red or green... If you use some filters that simulate colour blindness you will see that almost all types colour blindness result in this minigame breaking, or the green and red become hard to distinguish, or one of the two colours blend in the yellow background, the only people that can play this minigame properly while having colour blindness are those that don't see the blue colour...
The obvious answer is don't make any gameplay that relies on the ability to differentiate colours - e.g. in a match-3 game, make sure each colour of jewel also has a different shape.
In addition to using additional differentiating features beyond color (such as shape or text), using a different amount of color saturation in different colors will make them look different, even if the colors themselves appear the same.
You can use this palette from the article Color Universal Design (CUD) - How to make figures and presentations that are friendly to Colorblind people - by Masataka Okabe and Kei Ito.
Another option is to make the colors used by your game configurable in the options menu. That way everyone can figure out a set of colors which works for them.
The most common form of colorblindness you have to deal with is red/green. It effects ~7% of the male population in the US, not sure how well that holds up globally, as it's slightly more prevalent in Caucasians. The only other common form is also green oriented. The others are forms are often found in less than 1% of the population.
Unfortunately Red/Green are used pretty commonly to designate stop/go and bad/good culturally, at least in the US. The three best options as have been mentioned are to include shape differences as well as color, use different values/saturations, and most commonly avoid green as an identifier when possible. Hence the reason good guys & bad guys are often designated as Red vs. Blue in games.
All the information you need is here, together with examples:
http://gameaccessibilityguidelines.com/ensure-no-essential-information-is-conveyed-by-a-colour-alone
First port of call is to avoid relying on colour alone. Then test using a simulator for the most common types to pick up on contrast issues, e.g. red on black. If you're still unavoidably using colour, e.g team colors that need instant recognition, allow the colours of the relevant elements to be customised. Ideally free choice + presets for the most common types.
Colororacle.org has the most accurate simulator currently available.
Common mistakes to avoid -
Assuming CBness is about 'red-green' (which doesn't exist, it's two similar but separate things - red deficiency and green deficiency) or deuteranopia/protanopia/tritanopia, there are other and sometimes unique types, even different type in each eye, or whole ranges of colours swapped around.
Filters, the idea of a simple universal effect that will fix it all for you is a seductive one, but it isn't effective.
Validating with real people. This can't be done as CBness is so varied, and on top of that many people don't know what type or what severity they actually have. Instead, use simulators for validation, and real people as a way to pick up on extra issues you may have missed.
Again this is just a summary, the full info is at the above link.
