Excel supports a fifty-six color palette of which sixteen are used exclusively for Charts, and forty for cells in a spreadsheet. Colors can be defined by using RGB notation, and are shared among all worksheets in a workbook.
All colors displayed in the sections being exported are saved as a custom palette and saved in the HTML file (in a hidden xml section). By default Excel uses the color palette as a baseline for creating a custom palette. The colors that match Excel’s colors retain their positions. Unused positions are filled by default Excel colors.
Excel automatically determines which index in the color palette to use for a particular color occurring in CSS.
If the total number of colors is more than forty, Excel determines the color placement.