It's worth a look at Ilford's data sheet for variable contrast papers. They state that all of the emulsions are sensitive to blue and the varying contrast layers have varying amounts of green sensitising dye added.
When the paper is exposed to blue light, all parts of the emulsion react and contribute equally to the final image. This image is of high contrast because of the additive effect produced by three emulsions with the same speed and contrast.http://www.ilfordphoto.com/Webfiles/...8932591755.pdfWhen the paper is exposed to green light, only the parts of the emulsion with the larger amounts of green sensitising dye react initially. This is because the three emulsions have very different sensitivities to green light. This image is of low contrast because of the additive effect produced by three emulsions with different speeds to green light but with the same inherent contrast.
So a bit more complex than one emulsion sensitive to blue and another sensitive to green.