I typically have more art on-hand than frames because I have issues, so I figure out what size frames I need, and take a tape measure to the store with me. I look for frames that are in decent shape and try not to worry about the color. Sometimes I’ll even buy a frame just for the matte inside, because I like options.
Then at home I play musical frames until everything fits and complements the art. I make sure to clean the whole frame and both sides of the glass with vinegar. You cant usually tell, but trust me these things are filthy.