I suppose that you are using Unfold3D Real Space, and the RS version display the absolute stretching in UV space. Island are turning red since islands are scaled in a smaller dimension because of the Auto-Fit option, so that they are compressed. There is currently no option to change the stretch display. However, please read this below:
Usually, applications for industrial design need to work on the original (3D) dimensions of the mesh in UV space. However, when enabled, Auto-Fit option will rescale the island so that they fit in the current UV tile dimensions. So, the mesh original (3D) dimensions is lost when auto-fit is enabled.
Instead of enabling Auto-Fit you may follow that workflow:
1. open the UV Tile panel (1) and click on Fit. This will change the UV tile size instead of rescaling the islands.
If you want to export a rescaled version, without loosing the islands dimension, follow the next steps:
2. Open the exporter (2) 3. Enable the OBJ file export 4. Enable "Normalize" if you want your mesh rescaled into a normalized UV tile.
Having say that, I could add an additional display mode that shows distorsions color in a relative mode, like the Virtual Space does. Simply ask it, I will add it