Hi and welcome!
If you DO decide to go back to the store and get a muslin that matches the blocks you've already made, don't think of it as 'wasting' money, think of the 'first' muslin as 'adding to your stash.'
If the different weights and colors REALLY bother you, now's the time to do something about it. However, no one else is likely to notice, and you MAY be obsessing a little too much! I agree with Suz - different colors of the neutrals as well as the 'real' colors add to the charm of a quilt.