If they are tinted...I throw them out. Especially red parts. Red is the HARDEST color to match. I have 3 little part bins. ALL DIFFERENT SHADES OF RED or pink. Red mismatch tints show up more than the rest it seems. At least to me. You make a batch...one part...just ONE cast fails...its ALL garbage. I could try and match it up later...but that could take 3 or 4 or more attempts chasing a color for ONE part. Im better off throwing them all out and trying a complete pour all over. Im just wasting the mold among other things.
I have a whole tray of mismatched color parts...and the colors cant be matched up to other colors because the tints are specific. I could make a yellow and blue figure for instance, but the shade of blue doesn't look good with the particular shade of yellow.
I had thought of having a cast sale. 25 cents a part or something stupid crazy like that...but unless you have a rivet squeezer or all the other specific parts, my extra cast parts are no good to you.
So yeah...I have been throwing out the extra colored parts. Pure white, pure black and clear parts I do keep for later use because they always match up.