please empty your brain below
At that point in 2017, the specific shade of purple had been chosen (it changed with the line name reveal) and could have been gone with. Instead the station was given a modified version of the TfL Rail signs - the modification being no reference to TfL Rail as it would be confusing to do that.
Most of the East London stations only changed colour from blue to purple sometime between an April 2019 streetview car passby, and another passby about 18 months later. The others were behind hoardings. During that intervening period, those stations that changed had their platform lengthening, step free access and whatever works finished.
Southall went from NR signage to blue signs (the other West London stations skipped blue signs as the intermediate state was meant to be short) between March 2018 and May 2019. It still had blue signs in January.
Abbey Wood seems just following the same protocol for the TfL Rail -> Elizabeth Line transition: the signs are blue until the station is commissioned, when they become purple.
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