Summary buffer highlighting/colouring with Agent

[Richmond]

I am using xterm.

When I switch on Agent for a server I find the colouring changes in the
summary buffers. I think it is on all but one group. The colour has
changed to blue/grey as if the articles have been read, whereas they normally
appear in the foreground colour.

This also happens using gtk but the colours are different.

Why does it happen? What is the meaning of the colours? I haven't used a
face. I have a gnus-summary-line-format but it is just spacing etc.

(For those watching in black and white, the text that I am typing now is
in the foreground colour, but the header's names above are in blue/grey)