LEDs produce a very narrow spectral output; a bandwidth of only about 25nm wide for the most part. LEDs might be able to produce the centre-frequency of an ideal flowering spectrum and a few nm each way, but very little outside that narrow bandwidth, contributing to a completely insufficient luminous output overall.

Have a look at the broad spectral output of the sun (which is what we're shooting for), the narrower and peakier HPS (though which is reasonably well matched to the plant's flowering requirements) and then these sample red, blue and yellow LED spectral curves. Because of the narrow bandwidth of current technology LEDs, if you were replicating Old Sol or even HPS, several LEDs with slightly differing centre frequencies in each main color band would have to be used- and each would have to produce high luminous output, around 8-10K lumens each, to be of any effectiveness, even compared to CFLs.

If all that was needed to grow dope was light of the correct colour, you should reasonably be able to make up an image in Photoshop which is the correct colour and point your monitor at the plants... and I think we all know that won't work.

LEDs are a great technology. One of these days, someone will work out how to get the luminous ouput level and breadth of spectral output where they're needed. However, until then, LEDs are toys which won't actually grow you any appreciable amount of weed.

Experiment all you like- but by the time you get done sinking several hundred bucks into LEDs, you'd have had an HPS and a decent exhaust/ventilation system bought and paid for.