I tested 4k video with

--vpp-subburn filename="forced.srt"

and the text converted with wide attribute like

1
00:01:56,194 --> 00:02:00,326
<b>THE WEINSTEIN COMPANY
präsentiert</b>

is not opaque. The background picture shines through. White is not fully white.
On black background it is ok. Colorspace is HLG with
--vpp-libplacebo-tonemapping src_csp=auto,dst_csp=HLG
No colors possible. Latest working version 7.82.
Tested on NVEncC (x64) 8.04.