I don't think you can change individual headers unless you hide headings and use text object for them.
I had included two images for working with chart colors.
"Custom Format Cells" is available on right click if you enable the option Always Show Design Menu Items on the Design tab of the user preferences (in Settings Menu) window.
Second option is in Visual Clues Tab of chart properties.
Third option would be going to the expression tab; and writing expressions in definition box for the related attributes to change colors