I could be mistaken but as far as I know TMB-180 had issues with easy lost of collimation during traveling. And TMB-175 got different lens cell manufacturer and this problem was solved. Also 175 has superior color correction over 180. I never seen any traces of false color in-and-out of focus on my TMB-175 lens #26 made July 2003. Temperature stabilisation time - out of house with 73F inside to 16F outside at my backyard was 38 minutes.
I've seen deepsky images made by Michael Downing using TMB-180 and haven't see any trace of bad color correction. I've just started with deepsky photography and this is my very first image of M57 in orange-reddish skies over Staten Island - just several miles from Times Square 8) . I forgot to put IR-UV cut filter, but still see someting

. No false color

(monochrome image - I got color filter wheel just couple of days ago).