Duplicate of BUG-000162396: "Color values change when using CMYK mode." This behavior is expected because ArcGIS Pro is a color-managed application. In ArcMap, the values do not change because there is no color management in the system at all. In ArcGIS Pro, the color fidelity is preserved as much as possible by default. To enable color management to specify exactly how color transformations are handled instead of relying on the internal defaults. Specifically, take note of the Color Conversion Rule setting used to translate colors from one color profile to another. For more information, refer to https://pro.arcgis.com/en/pro-app/latest/help/projects/color-management-application-settings.htm.