Hi Kristin,
You can export your out of the box chart and then color code in XML file. Then reupload with the same name overwriting the existing file.
You need to change the RGB in the following snippet of XML.
<Chart Palette="None" PaletteCustomColors="91,151,213; 255,0,0; 255,255,0; 255,192,0; 68,114,196; 112,173,71; 37,94,145; 158,72,14; 117,55,125; 153,115,0; 38,68,120; 67,104,43; 124,175,221; 241,151,90; 186,144,192; 255,205,51; 105,142,208; 140,193,104; 50,125,194; 210,96,18; 150,83,159; 204,154,0; 51,90,161; 90,138,57;">
<Series>
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-
-
-
</Chart>
Thanks,