This pane is part of the Preferences window which is shown when you select the Preferences menu item. It allows you to control the colours used to show errors, warnings, and summarised lines in the log window.
The colours are organised by the error severity: information, warning, error, and summary. For each of these there are a pair of colours, background and foreground. Each colour has two controls:
A text field which shows the hexadecimal value of the colour. You can type hexadecimal colours directly into this field. If the text field is empty, the default foreground or background of the GTK theme will be used.
A Choose... button which when clicked pops up a standard GTK colour selection dialog. Pressing OK in that dialog writes the hexadecimal value of the currently selected colour into the text field.
The Colours pane also contains a Sample area which shows you how the currently selected areas will appear in the log window, and these buttons: