Colour Psycology

RED. Physical
Positive: Physical courage, strength, warmth, energy, basic survival, 'fight or flight', stimulation, masculinity, excitement.
Negative: Defiance, aggression, visual impact, strain.

Being the longest wavelength, red is a powerful colour. Although not technically the most visible, it has the property of appearing to be nearer than it is and therefore it grabs our attention first. Hence its effectiveness in traffic lights the world over. Its effect is physical; it stimulates us and raises the pulse rate, giving the impression that time is passing faster than it is. It relates to the masculine principle and can activate the "fight or flight" instinct. Red is strong, and very basic. Pure red is the simplest colour, with no subtlety. It is stimulating and lively, very friendly. At the same time, it can be perceived as demanding and aggressive.

BLUE. Intellectual.
Positive: Intelligence, communication, trust, efficiency, serenity, duty, logic, coolness, reflection, calm.
Negative: Coldness, aloofness, lack of emotion, unfriendliness.

Blue is the colour of the mind and is essentially soothing; it affects us mentally, rather than the physical reaction we have to red. Strong blues will stimulate clear thought and lighter, soft blues will calm the mind and aid concentration. Consequently it is serene and mentally calming. It is the colour of clear communication. Blue objects do not appear to be as close to us as red ones. Time and again in research, blue is the world's favourite colour. However, it can be perceived as cold, unemotional and unfriendly.

YELLOW. Emotional
Positive: Optimism, confidence, self-esteem, extraversion, emotional strength, friendliness, creativity.
Negative: Irrationality, fear, emotional fragility, depression, anxiety, suicide.

The yellow wavelength is relatively long and essentially stimulating. In this case the stimulus is emotional, therefore yellow is the strongest colour, psychologically. The right yellow will lift our spirits and our self-esteem; it is the colour of confidence and optimism. Too much of it, or the wrong tone in relation to the other tones in a colour scheme, can cause self-esteem to plummet, giving rise to fear and anxiety. Our "yellow streak" can surface.

GREEN. Balance
Positive: Harmony, balance, refreshment, universal love, rest, restoration, reassurance, environmental awareness, equilibrium, peace.
Negative: Boredom, stagnation, blandness, enervation.

Green strikes the eye in such a way as to require no adjustment whatever and is, therefore, restful. Being in the centre of the spectrum, it is the colour of balance - a more important concept than many people realise. When the world about us contains plenty of green, this indicates the presence of water, and little danger of famine, so we are reassured by green, on a primitive level. Negatively, it can indicate stagnation and, incorrectly used, will be perceived as being too bland.

SRC:http://www.colour-affects.co.uk/psychological-properties-of-colours

I would use red if i was low on health id flash the colour to make sure the player is well aware that they are about to die, the positives to red are being health packs, usually red can symbolism red. Blue would genuinely be used for investigating something to light up an area that may need looking at or simply something slightly hidden, In anime style games blue would represent intellect so if a character had blue hair they genuinely be smart. Yellow would be more to signify importance or maybe a yellow exclamation-mark for a quest,yellow is also good to catch the attention of players. Green to me would more be health packs and life healing which could also be red too, but genuinely health on the other hand it could signify poison.