I've read somewhere that the most scientific and reliable approach to mushrooms identification is by comparing their spores. You'll need a microscope for that, however it seems more doable than using a color measuring device. By the way, it's quite easy to make such a device. Just use a digital camera with fixed power flash and manual (not automatic) presets options for white balance, aperture, and shutter speed to photograph mushrooms from a certain constant distance. And then use some graphics editor to extract and compare the color components values (e.g. in RGB) from a slightly blurred images. However you are on your own in building of the comparison / identification chart from scratch with this method.

IMHO, the best way for the field identification would be a step by step features separation guide, like one I saw for trees and another - for insects (where you have to go through the enumerated list of questions regarding the observable features of a specimen, while determining the number of the next question from the answer to the current one). Is there something like that available for the mushrooms? If not, it should be a great opportunity for a bestseller book.