You can customize almost all the colors of the chart generated by WebChart platform now. In previous release, it's not possible to define the line color or bar color, it can only use the pre-defined colors. In latest release, if you did not customer the colors, then the same behaviour with previous release, else use the customized colors. Take a look at the following chart sample.
Following lines are the WCML file defination.
WEBCHART.QUERY_1=select trade_month, trade_count/10000 \
I should ask a prefessional UI designer to choose the default colors, not choosed by a programer.