I've been doing contact printing on multigrade papers, but just using a light bulb for illumination (I don't have an enlarger, nor the space for one, nor the all-important spousal permission for one). As far as I know, this generally means I'm getting grade 2, which is usually fine. However, I'd like to have the other grades available, which means filters.

The obvious way to do this, I think, is to get a big gel-type contrast filter and put it on top of the printing frame itself. But as far as I can tell, no one makes contrast filters in a size big enough to do that (I'm mainly printing 9x12cm).

Of course, I could always get some coloured glass or theatrical gels or something in the proper colours, and there I'd be with perfectly good contrast filters, right? Only I can't seem to find out what "the proper colours" are. Is there a spec sheet somewhere that says exactly what colour a filter should be for a given contrast grade on VC paper?