Will Thiourea Dioxide Affect the Color Shade?

It can, depending on the dye and process. That is precisely why Thiourea Dioxide should be evaluated using instrumental color measurement.

Recommended measurements: ΔE, K/S, reflectance, whiteness index and wash fastness.

For polyester reduction clearing, the desired outcome is usually removal of unfixed/surface dye while preserving the intended shade.

Reduction clearing is specifically intended to remove surplus/unfixed disperse dye and improve fastness.