either way, you'll have to buy something, jennifer. if you have enough of that blue paint left, you can take it back to the paint store and have them mix in some grey (white will lighten, not mute). then test on a big board of plywood. hold it up to your wall. if it is still too bright, take the paint back and have it muted by a bit more grey. Or buy the grey paint yourself and try mixing the two in a fresh empty can (try 1 part grey to 6 parts blue first). that way, you have more control over the color.