Summary:
In Bedrock Edition since version 1.18.0.20 the empty part of a boss bar is always grey (at all boss bars). According to MCPE-161436 and ADO 883959 this is an intentional design change. So this is now a valid parity problem.

Steps to reproduce:

1. Summon a Raid/Ender Dragon.
2. Make the Boss Bar not full.

Observed results:
Boss Bar in the empty part is dark red (Raid)/dark pink (Ender Dragon).

Expected results:
Boss Bar in the empty part is gray instead of dark red (Raid)/dark pink (Ender Dragon).

Screenshots:
Java

Bedrock