Changing the color of an existing item(like a set of armor) is called re-texturing.
The most common programs used to create textures for Fallout 3 are GIMP and Adobe Photoshop. GIMP is free... and Photoshop is expensive but perhaps more intuitive to use. Both are fully capable of creating content for the game. You will need to spend time learning how to use one of these programs if you plan on making your own textures.
Textures for the game need to be in the .DDS format. In order to create/save textures in this format, you will need the DDS plugins for either GIMP or Photoshop. These plugins are free and easy to find using google.
You may also want to learn how to use a program called Nifskope... which can help you apply your textures to the models used by the game. However, depending on how you set up your re-texture... you may not even need Nifskope. For example, if you create a texture for the t-51b armor and name your texture files exactly the same as the originals... and place them in the exact same directories as the original textures.. then they will overwrite the original textures.... and you wouldn't even need to create a mod(.esp) with the GECK. The downside to this is that all instances of that armor used in game would use your custom texture... which may not be what you want.
Nifskope offers more flexibility because it allow you to specify the texture directories used by a model.
As far as where to look for custom textured t-51 armor... the http://www.fallout3nexus.com/index.php is always the best place for FO3 mods. I can't say if you'll find what you want though... and it might be more rewarding creating it yourself.