Gold at top of plant

Hi,

Pics came out rubbish, cant see it..

Anyway I have 6 plants in one tent, bout a foot an ahalf tall..

3 of them, have got gold on top of the plant, on the leaves.. I say gold because it looks more like gold than yellow, ha..

Anyone no whats causing it??

Using 600 watt light, an I have just moved them up a bit more to make sure am not burning them alive, ha, hopes its this though easy to solve.. ha

thanks people