My wedding colors were red and orange so we decided to go with red LEDs. My advice is to basically get the brightest LED you can find that is closest to your color. The brightness of LEDs is measured in millicandela (mcd) however a higher mcd does not necessarily mean a brighter light because it is dependent on the viewing angle which determines how focused the beam of light is. I found a great page
that explains all of this and halfway down even has a calculator so you can enter in the mcd and viewing angle of the LEDs you are looking to purchase and get an idea of how bright your LED actually is.
I used red LEDs with a beam angle or 55-65 and an mcd of 1000-2000 and they were very bright. Basically you will want to get the brightest LED you can with a relatively wide angle. A smaller angle means the middle of the card will light up more than the edges while a wider angle disperses the light better. I did find some manufacturers claimed theirs were brighter than they actually were, so you may want to get your LEDs early just to test them out.
Also, and I can't remember where I read this, but I did find a site that basically said red LEDs will burn through the unregulated battery faster whereas other ones will stay brighter longer. I had to install pull tabs to activate the cards soon before the wedding so they'd still be at full brightness by the time of the reception. For some other colors you may be able to activate them the night before and not lose much brightness by the next day. Get your LEDs early and test one overnight to see how well it holds its brightness.