I was able to found out how to change the DEFAULT color with the following code:
colorPalette : ["orange","red"] .
The problem now is that the system only takes the FIRST color set, in this case "orange" for the Column chart. If it were a pie chart, BOTH colors will have been considered. Does it mean that we CANNOT set different colors for the bars in the COLUMN chart? I searched through the net and I have the impression that this is the case for the Column chart, which means that ALL bars will have the SAME color. Can someone please confirm it, or tell me what to do to have DIFFERENT colors for the bars in the Column chart? I will appreciate any hints, thanks in advance 
