Blade colors can be tricky. I assume you already have the color .tga and .txi (should have a name like w_lsabre(color)###.tga). You won't need to change the .txi to change the blade color.
The K1 blades have an inner circle with a verticle white stripe and an outer 'glow', like so:
http://www.princessartemis.com/remote/saberblade1.jpg)
http://www.princessartemis.com/remote/saberblade2.jpg)
Most K2 blades do as well. What you can do to change the color is use a graphics program that can handle .tgas (Paint Shop Pro, PhotoShop, and Painter do; I have experience with them... but I understand GIMP does these things as well--unfortunately I don't know how to use it, but these instructions should help in any case.)
Take the .tga, and without changing the lightness, you can change the hue and saturation of the color. There's usually an option to do this under menus like Effects --> Tonal Control --> Adjust Colors for Painter, Colors --> Adjust --> Hue/Saturation/Lightness in PSP, and Image --> Adjustments --> Hue/Saturation in PS. Anyway, whatever program you're using, find the controls for Hue, Saturation, and Lightness. Change the hue and saturation until you like the colors, and save the file. Drop it in Override and see how it looks on the 'saber. You'll have to do a lot of tweaking to get just what you want.
Fortunately, K1 is very good about letting you alt+tab out of it so that you can continue tweaking the .tga and putting it into Override where it will immediately update the texture. That way you can see right away how your new color will look in-game.
The white bar in the middle of the circle on the .tga causes most of the brightness in the blade, as well as the 'trail' when it is swung. You can carefully paint over the edges to darken the blade, or remove it entirely to further darken the blade (in some cases). If you want a blade with a black trail and a dark center, make the line black.
Another thing you can do is take a selection tool in the program you are using and select the center circle, then use Hue/Saturation/Lightness to change the color. (It's much safer to change the lightness of the center than the glow--that won't cause problems with the shape of the blade.) This can do some really interesting things to your blade by making the center an entirely different color than the glow. For instance, this, which I did by turning saturation all the way down for the whole image and then painting a dark red circle in the middle:
http://www.princessartemis.com/remote/saberblade3.jpg)
will make a blade like this (the 'Hematite' blade) in K2:
http://www.princessartemis.com/remote/saberhem.jpg)
(Please don't use this image to make a lightsaber blade--as you can see, it's a mod that I'm making, but you're welcome to duplicate the effect.)
So, there are a lot of options. Tinker around and see what works best for what color you want. You can also change the shape of the center; I haven't tried that myself, but I hear it is effective for some colors. (Edit: See what Shem did? Changed the shape of the center to make it bigger--it makes for a very nice, movie-style blade.) Just be careful about making the glow too bright or too big--you might end up with a very weird looking 'saber blade.
Just for completeness, I know you've looked up the tutorials for making lightsabers, but, I like being thorough:
http://www.lucasforums.com/showthread.php?s=&threadid=143456) has files indespensible for making a lightsaber (some names are a tad different for K2, but it's not hard to find what you need) and
http://www.lucasforums.com/showthread.php?t=143991) has a tutorial for making a 'saber upgradeable. Also, if you make a custom blade, you can use K-GFF or another GFF editor to delete the UpgradeLevel struct from the bottom of a K2 lightsaber so that it can't be changed--this should prevent disappearing 'sabers and sometimes game crashes.
I know that last bit was more than you were looking for, but I hope the first part helps :)