I noticed this happening in my Konsole terminal windows very occasionally over the last couple of months: a little coloured square with a black border would momentarily appear, then disappear when I moved the mouse. I never quite figured out what was going on, thought it was a window manager glitch or something ... until today. It appears Konsole has this (undocumented?) feature where, if you hover over the name of a colour in a terminal session, it will helpfully pop up this square showing what the colour looks like. The colour names come from the traditional X11 set in /usr/share/X11/rgb.txt, with limits: it only looks at single-word names, and does not match names with numeric suffixes. Thus, “darkmagenta” will work (case-insensitive match against “DarkMagenta”), while “dark magenta” only matches the “magenta” part, and “magenta1” is not recognized at all. It will also recognize a sequence of 3 or 6 hex digits preceded by “#” -- “#rgb” or “#rrggbb”, as commonly used in HTML. Just tried 8 hex digits -- “#rrggbbaa” with alpha transparency -- and that works too. This feature might be new in Konsole 21.08 or something.