To examine items, you can throw them out of your bag and then press up while standing over them. However, the color doesn't matter, so you should be able to just recognize them by their shape. Basic Shrooms are the common ones you find everywhere in Mushroom Zones, Wide Shrooms are twice as wide as Basic Shrooms, Tall Shrooms are twice as tall as Basic Shrooms, and Tiny Shrooms are very small (about 1/4th the size of Basic Shrooms). Early on in the game it is difficult to find anything other than Basic Shrooms, so until you get farther in the game, you will probably mostly see the other types of shroom from the person in the Citey who sells uncommon mushrooms.