Just out of interest for those who want to go over the colours they're interested, here's a bit of background info on LED's and what they're made up of to produce the different colours in the spectrum.
http://www.oksolar.com/led/led_color_chart.htm
The reason why we can't have pink or brown yet for example is because the material used has a certain electron value in it's atomic structure. If you've done basic chemistry, you'd know that when atoms get agitated, their electrons get all excited and start jumping to outer shells orbiting the eletron (I'm being 'lamens terms' and 'liberal' about this). This energy is given off as light of different values in the spectrum, mostly what we can see. Most people already know about the 'Periodic Table of the Elements' that funny looking graph with acronyms like Si (Silicone), Fe (Iron), O (Oxygen), etc... we only have a certain amount of different elements and we can only use certain elements in an LED to make certain colours. You can see in the table from that website that the difference of making a blue LED to a green one is that:
Blue has: SiC/GaN - Silicon Carbide / Gallium Nitride
Green has: InGaAIP - Indium Gallium Aluminum Phosphide
It's like when you put certain metals (the periodic table mostly is transition metals) like copper for example and burn it in a flame, the flame goes green. You're actually seeing electrons going crazy because of the temperature. The electrons eventually do go back to their original stable orbits around the atom and that's when the burnt copper atoms escape the flame, oxidise and are now part of the atmosphere. (Hope you're following along...
). This all happens in a split second by the way.
I believe (given time) we can develop LED's to produce any colour in the visible spectrum of light just using the elements. In the meantime if you want let's say pink, you can get LED's that emit a bright white light and they have a pink filament cap around the housing. Fairly effective but whatever floats your boat.
If you want a purple LED however, it is possible, they have been going around recently. They are expensive though because it's still considered 'new' technology. If you want to see a pure Violet LED in action head here:
http://ledmuseum.home.att.net/ledvio.htm
