The red underside was depicted on the box for the model. But there were nostalgic factors involved here, too. The second plane I ever did when I was a kid was a Hasegawa 1/32 FW 190. I think I got it at Toys R Us for about $5.00 in the mid 70's. I built it thinking that I was doing a much better job with the FW 190 than what I did with my first model, a Revell P-40. It didn't dawn on me on that time that the pieces fit much better on that kit. Anyway, I built it and Dad painted it with a red underside and hung it in my room. Dad requested a FW 190 so thought it might be kind of neat to give it a red underside on this one.
Learned a bit on mottling and other things....so it's all good!