OK, once and for all, I will post what I hope covers all of the bases as pertaining to the colour of semen - they will be differentiated by maturity, diet, and frequency of masturbation.

Thin/watery/clear
This kind of ejaculate is mostly found in people who are very new to puberty (as prior to puberty, people don't tend to masturbate and if they did, there'd be no ejaculate anyway). The ejaculate might be clear because that is the colour of some people's normal semen (without any sperm). You might also note that the colour of pre-ejaculate is very similar to that of this immature ejaculate. Although sperm can be said to have "been in production" since you were very small, they are only mature enough at the onset of puberty and this is when you'll start to see the colour change.

If you have started puberty and are ejaculating on a regular basis, but still find your come to be of this colour, do not panic. It's either that you're masturbating too often or else your semen and sperm production is just a bit slow. It's not a problem - some people go all their lives with thinner semen and are fine.

Thick/white
The kind most people have when they are fully matured. This kind of colour denotes that your ejaculate is mature. If you ejaculate this colour on a regular basis but find sometimes the colour goes back to the one discussed above, that's fine - you're, again, either masturbating too often for the sperm production to keep up (no issue), or you're drinking a LOT of fluid. Remember, sperm takes about 70 days to go from zero to hero!

Yellow
A fine colour; perhaps slightly tinged by urine, or reflecting the light strangely. Could also be something you've eaten or your natural colour - it's nothing to worry about. Most often found in older people.

Brown
Probably indicates an infection or old blood in the urethra. Seek medical attention.

Red
Indicates infection, trauma, or another medical complaint. Seek immediate medical attention.

You can't really change your colour to any major degree, and you shouldn't bother trying because it's pointless. Also remember that with the change of colour normally comes a change in consistency: the clearer will be more watery, whereas the white or yellow will be thick, viscous and gloopy. A jelly type is fine, that's because the semen 'solidifies' within a few minutes of being ejaculated (and some may have been left in the urethra previously).

Something else to bear in mind: Anti-biotics, LARGE amounts of food colourants, and some other drugs can affect the colour of your semen. Also, just to finish this off nicely: Sperm can be affected by alcohol, smoking and environmental factors such as radiation and toxins.

SOOOOO, to conclude:

White: Fine.
Yellow: Fine.
Clear: Fine.
Red: Bad.
Brown: Probably OK, but get it checked.

Remember: Sperm only makes up about 1% of semen.

Semen
Simply put: The carrying fluid for the sperm.

Sperm
The little buggers that can make babies.

Pre-ejaculate
Comes BEFORE orgasm and clears out the urethra of any acidic properties, this helps the sperm survive when being ejaculated. Can sometimes contain the semen (and thus sperm) of previous ejaculations if there has been no urination beforehand.