A monochrome filter is a filter made from a piece of plastic in a primary color. Looking through it at objects reduces the colors you're seeing to tones of whatever color the filter is, enabling you to easily see and compare how light or dark the tones or values are.
Dark red is generally used for a monochrome filter, but if your subject's got a lot of red in it, try a strong blue.
The easiest way to make one is look in a craft supplies store or stationer's for a plastic folder or file divider in a suitable color. Cut it to smaller size if you prefer, and stiffen the edges by gluing a piece of stiff card onto it. Or paint up a piece of stiff plastic or glass with a transparent red.
