Caffenol can produce staining (especially if you use a formula that lacks ascorbate) that can even mimic a color negative. I have a roll of scans of 35 mm Tri-X (from 2003) that I scanned as color and look very much like 2-color Technicolor -- reds and greens and their combinations, but no blues. As others have noted, the antihalation in 120 Fomapan is bright blue/teal when it pours out, and I've never seen it stay in the film -- but I haven't developed Fomapan in a while; it may have changed. A combination of the reddish/brown stain from Caffenol and a tiny bit of residual blue AH would give a purple tone.
Alternatively, could you have gotten a T-Max in place of Fomapan by mistake? Those are famous for pink base, especially if fix in non-rapid fixer. Ilford XP2 Super also has a pink cast, and develops fine in B&W developers (I haven't tried it in Caffenol, however, so I don't know what staining effects it might get).