Hi Gisele,
I am surprised nobody got back to you. This is something we've done on many installations of Moodle. It is just a PHP coding customisation rather than a Moodle thing.
You can simply change the following line in graph.php:
$Test->createColorGradientPalette(195,204,56,223,110,41,5);
Change it To:
$Test->createColorGradientPalette(249,186,17,166,6,195,0);
That will give a little more variety to the colours, rather than the many shades of yellow and orange. It should be as simple as that.
I hope that helps.
Cheers,
Richard