使用ggplot2绘制图形时，为data_frame中的不同级别的列选择特定颜色

I'm using ggplot2 to make several graphs from a large data frame. The graphs are bar graphs showing proportions of two levels of a column in the data-frame, Yes and No. The colors are chosen by default, and want to make one of the levels either completely transparent, or choose a different color for it. to make several graphs from a