Background color. All CSS Color Module Level 4 formats should be valid. See https://www.w3.org/TR/css-color-4/
Which callsign to highlight
Foreground color. All CSS Color Module Level 4 formats should be valid. See https://www.w3.org/TR/css-color-4/
The "Highlight last" field allows the sender to request that all instances of "Callsign" in the last period only, instead of all instances in all periods, be highlighted.
WSJT-X client name
Whether to reset the highlighting to default (overrides background and foreground colors)
The server may send this message at any time. The message specifies the background and foreground color that will be used to highlight the specified callsign in the decoded messages printed in the Band Activity panel.
See WSJT-X source.