I like this Brick Texture method more than the Noise Texture method.
But since the brick texture interpolates between two colors --instead of the individual color channels-- I would use 2 brick nodes (one for U, one for V), to have it going random on both axes independently (and one of the axis offset by one or more whole units). It should allow for more randomness.
The brick texture is also infinite, so another way to add a random control is by offsetting the original UVs that plug into the brick textures, like so:
Edit: In this image the CELLS value should be 5 and not 4, since it’s using a 5x5 atlas image.
The Converter→Map Range node seems to be from the Blender 2.8x series. If you’re on Blender 2.79 you’ll have to replace it with something like that stepify group.
