Try to avoid adding texture that's made of single pixels, as that looks like noise. Use shapes made out of many pixels (4+) to get a camo pattern going, then add brown blobs of rust. Your camo pattern might be unrealistic large, but it's more important that these things read well to the viewer. Realism means nothing if you fail to communicate your intent to the viewer.
For the rust, use darker browns, and don't give them a grey outline, the outline makes them not look like rust, and takes up valuable space you can use for the rust (the larger a shape is, the easier it is to discern what it is).