Final answer:
The keyboard shortcut to fill color in Excel is Alt + H, H, which opens the color palette, and F4 can be used to apply the last used color to new cells.
Step-by-step explanation:
The keyboard shortcut used to fill color in a cell in Microsoft Excel is Alt + H, H. First, select the cell or range of cells where you want to apply the fill color. Then press and hold the Alt key. While holding this key, press H followed by H again. This will open the color palette. You can then use the arrow keys to select the desired color and press Enter to apply it. If you want to apply the last used color, you can simply press F4 after selecting new cells to replicate the last action.