Steps to split a cell diagonally and fill only one half with color in Microsoft Excel:

1. Select the cell that you want to split diagonally.
2. Right-click on the cell and select "Format Cells" from the drop-down menu.
3. In the "Format Cells" dialog box, select the "Border" tab.
4. Under "Presets," select the diagonal line that you want to use to split the cell.
5. Click on the diagonal line to select it, and then click on the "Color" drop-down menu to choose the color that you want to use for the line.
6. Click "OK" to apply the diagonal line to the cell.
7. Now, select the half of the cell that you want to fill with color.
8. Right-click on the selected half and choose "Format Cells" again.
9. In the "Format Cells" dialog box, select the "Fill" tab.
10. Choose the color that you want to use for the fill, and then click "OK" to apply the color to the selected half of the cell.

That's it! You should now have a cell that is split diagonally with one half filled with color.