Not so much stained...

A filter is, think camera lens filter, a really, really thin paint that when applied to a large surface shifts the tone of the base coat.

So if you have a green tank and the green is monotonous you can apply a....say yellow filter, and that layer of really thin yellow paint will cause the green color under it to shift a bit, adding subtle variation.

I don't have an example to post at the moment,but that's the basic idea.