Aleksandar,
I tested in your group and you can see.
I do indeed.
And I'm more convinced, looking at the source, that it came from your email interface. The boundary marker that separates it from the message bodies:
Content-Type: multipart/mixed;
boundary="_B716E74D-7A6D-4E1A-A7AE-430B2B75007D_"
is of the same format as the marker that separates the message bodies (plain and HTML) from each other:
Content-Type: multipart/alternative;
boundary="_945C41F1-806F-41E8-BE8B-D84011A2A67E_"
That seems an unlikely coincidence for two separate origins.
I was wrong about the color. it isn't white or transparent, it is a pale steel blue.
To really nail this, send another reply to the test group but this time cc me directly. Then I'll be able to compare the via-group copy with one untouched by Groups.io. If you'd like to see also CC yourself at another address (NOT the same gmail address as you're sending from).
Shal
Shal Farley