Please give a warm welcome to Eva Levante, the new Guardian outfitter. You can visit her in Tower North to buy Shaders for 2250 Glimmer each. Use the shader by right clicking with it in your hand, then doing it again to confirm that you wish to apply the shader. It'll color your armor according to the shader icon, but beware. The shaders are one time use, and are consumed when you apply it, so make sure you use it on the armor you're going to use for a while.