diff --git a/doc/config.but b/doc/config.but
index db0369b6..d24aaf17 100644
--- a/doc/config.but
+++ b/doc/config.but
@@ -1409,6 +1409,20 @@ This option is enabled by default. If it is disabled, PuTTY will
 ignore any control sequences sent by the server which use the
 extended 256-colour mode supported by recent versions of \cw{xterm}.
 
+If you have an application which is supposed to use 256-colour mode
+and it isn't working, you may find you need to tell your server that
+your terminal supports 256 colours. On Unix, you do this by ensuring
+that the setting of \cw{TERM} describes a 256-colour-capable
+terminal. You can check this using a command such as \c{infocmp}:
+
+\c $ infocmp | grep colors
+\c         colors#256, cols#80, it#8, lines#24, pairs#256,
+\e         bbbbbbbbbb
+
+If you do not see \cq{colors#256} in the output, you may need to
+change your terminal setting. On modern Linux machines, you could
+try \cq{xterm-256color}.
+
 \S{config-boldcolour} \q{Bolded text is a different colour}
 
 \cfg{winhelp-topic}{colours.bold}