We have a rounding bug in the server, causing excessively distorted colours when in low colour mode. See the following upstream report: https://github.com/TigerVNC/tigervnc/issues/605 A fix is available upstream: https://github.com/TigerVNC/tigervnc/commit/816baa35ae5374f350ee4031ac54d7d1e90af183
Vendordrop of #7158 seems to fix the problem with shifted color in reduction.