I bought 3 male guppies yesterday, a blue, yellow and an orange coloured one. The orange one this morning is not very active and is turning a grey colour at his tail. The other 2 are swimming round ok, any ideas?
Water quality problems are often the source of most tank worries- you should buy some test kits for ammonia, nitrites and nitrates so you can test your tanks state of water quality . You need to give more info on your situation- although the guppys may have died, whatever killed them could still be an issue in the tank.
How many gallons is the tank? How long has it been set up? What sorts of fish & things do you have in there or how many plus what temp is the tank?