A set only works if the top and bottom look like they belong together. Hoodies and sweatpants are often cut from the same fabric, but body fabric, rib and cuffs can still take color differently, and two pieces dyed at different times can drift apart. Developing both garments in one project — same fabric source, colors approved side by side — is the most reliable way to keep a set matched.
Decide early whether the set is strictly matching or deliberately tonal, and whether branding should mirror across both pieces.