Mushroom spore colour is often an essential step in species identification, and creating a "spore print" from a specimen is one way of determining the colour.
Spores may be dark or light in colour, and so it's often useful to make a spore print on both black and white surfaces. The MST has made available a PDF document for printing your own spore-print paper or cards.
Spore print card template © Mycological Society of Toronto and licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.
Remove the stem of the specimen and place the cap of the mushroom on the spore print card with the gills (or pores) facing downward. Cover it or leave it undisturbed overnight and compare the colour of the resulting print with descriptions in your field guide.