Hi Lynsey
The settings for graph colours are controlled through the parameters added into the graphing tool rather than through changing CSS.
The setting you want to input should be added to the custom setting in the report builder graph area
Depending on the data and type of graph you can change the colours of each data item by adding the string listing the colours required separated by a comma e.g.
colours = #ff0000,#00ff00,#0000ff
regards