paphiopedilum - leaves turn red

[christoph0315]

Hey y'all!
One of my Paphs, Paphiopedilum hirsutissimum to be precise, is giving me this picture:








Overall the plant looks happy to me. I admit I don't water them too often. They are, as most of my Paphs in relatively loose bark mix, Bonsai Jacks universal potting mix. I dont give much fertilizer but if i do, its either 20-20-20 or 13-20-10.

any thoughts what could have caused this or all good?

Thx!