after i've used wilton to tint icing or fondant the color seems to darken over night.
is it my lousy memory or does this really happen?
I love your wording and, yes, it definitely happens. I haven't noticed a drastic change myself but many do darker a little.
When you color BC, the color will often dark or deepen a shade after sitting. I usually tell my students to try to make their icing at least a few hours in advance, so they can check the color before they start decorating. (I don't notice it w/ RI or fondant. GP will actually lighten as it dries.) HTH!