Tamiya's decals are sometimes thick, and would be opaque. I have a tendency to not trust decals for things like that. My approach would be to paint the lower fuselage white, mask off the white stripes with low-tack tape, and then apply the RLM 23 Rot. If you paint the undersides red, and you don't like the way the decals look, trying to cover it with white will be very difficult.
Hope this helps.