It should be read on the blue chanel. Usually you will see about a stop difference (0.3) between the red channel and the blue channel. This is a fairly accurate number for regular enlarging paper, for UV sensitive materials like pt or azo the response is not as accurate and you see greater response on the highlights which have more UV blocking response.

The stain color is fairly accurate in the sense that brown negatives have a greater UV blocking response than pyro negs, but both are fairly close.