If you don't need it done in real-time, then you're better off using Photoshop or GIMP to save two copies of your image.
If you need it in real-time or need to process a lot of sprites, using shaders is the ideal solution.Shader Lesson 3: Grayscale and other effectsCode example
Another solution is to process the Pixmap before uploading it to a Texture:Pixmaps & Textures
Yeah it needs to be done in real time. So I'm going with Shaders. I found settings for a default shader as well.
My problem right now is that when I apply the grayscaled shader nothing renders... but when I set the shader to the default it renders everything after it fine.
I'm, obviously, very new to shaders so is there anything simple that I might be missing?
Note: I copied VERT and FRAG entirely from the code example.
This is what takes place in my render...
time += Gdx.graphics.getDeltaTime();
grayscale = (float)Math.sin(time)/2f+0.5f;