Illustrator color issue

so i haven’t really used Illustrator in awhile, well about a year, and i recently opened it up. after a few hours playing around with things i’m picking it up again but am running into a little snag here and there.

my main problem and reason for posting here is that i’m having some trouble with changing the color of objects. for example: i’ve got a shape on the artboard and if i wanted to change the color of it i would select it, and then pick a new color then it would be applied automatically, but what i’ve been running into is when i select the object, i pick a new color and nothing happens… i dont know if i hit a keyboard shortcut or something but this is getting annoying. if i have a brush stroke on the artboard thats black lets say, and i want it to change it to green, i selected it, picked a green color and its still black…

anyone got any ideas/tips?