What I’ve learned so far is that colors are deceiving – they depend on geographical location and position under sun (or shade), wether it’s moist or dry, camera settings etc. I compared Sharnoff’s pages on both species side to side, but the pictures are too small there, so I went to Stridvall, and his pictures clearly show labriform soralia in P.dubia, while P.caesia has soralia looking like pompouns, similar to these photos. Both species seem to have specimen with soralia far away from center almost to the outer perimeter of the rosette. So I’ll stck with P.caesia here, until I’ll grab some specimens next season.

But caesia tends to have a bluish tint to the soralia. One source says that dubia tends to have soredia mostly at the lobe tips, while caesia has them mostly on surface of the lobes more toward the center. Another source calls dubia’s soralia “labriform” or lip-lik, at the lobe tips.

Yours has soredia near the tips, but definitely not labriform. Stick with caesia despite the lack of blue color?

Due to my very limited exposure to these two species, I don’t yet know how to tell them apart.