diff --git a/session_4/session_4.Rmd b/session_4/session_4.Rmd
index 1b572b6468b6c83d61da0de6cec4d7ab1995013e..2b29250fa375aa36fa3a9f69cd47610012592e81 100644
--- a/session_4/session_4.Rmd
+++ b/session_4/session_4.Rmd
@@ -503,4 +503,10 @@ ggplot(expr_DM1, aes(samples, Genes, fill= log1p(counts))) +
   </p>
 </details>
 
+With the default color gradient, even with the transformation, the heatmap is difficult to study. 
+
+R interprets a large number of colors, indicated in RGB, hexadimal, or just by name. For example :
+
+
+