3 ways can do color:

1. Color Palette – The default color palette is – Default UI14. This is a reference field pointing to Chart Color Schema [pa_chart_color_schemes]
2. Chart Color – We can select chart color, this is a reference field pointing to Chart Color [sys_report_chart_color] table.
3. Use Several Colors: Example Hexa Color Code: FFD700,#DAA520,#F5F17E,#CCCC00

Both tables point to Color Definition [sys_report_color] table.

Important Table :

1. Chart Color Schema [pa_chart_color_schemes]
2. Chart Color [sys_report_chart_color]
3. Color Definition [sys_report_color]

For more information: