We feel the same way and actually started doing this with colors. There's a guideline section in the app where designers create a color palette, define all the colors that they'll be using. This way, when you're looking at the designs, you can instantly see if there's a color that is not defined in the palette.

We've heard back from a lot of designers using Zeplin saying that they're now more cautious about which colors they use. We're now working on a similar feature for reusable UI components.