Yes, there is a way to force the grid lines to show even when there is a fill color applied to a block of cells. Here's how you can do it:

1. Select the block of cells where you have applied the fill color.
2. Go to the Home tab in the ribbon at the top of the Excel window.
3. Click on the Format button in the Cells group.
4. In the dropdown menu, select Format Cells.
5. In the Format Cells dialog box, go to the Border tab.
6. Under Presets, select the type of border you want to apply to the cells. For example, you can select All Borders to apply a border around all sides of the cells.
7. Click on the Color dropdown menu to select the color of the border.
8. Click on the OK button to apply the border to the cells.

By applying a border to the cells, you can force the grid lines to show even when there is a fill color applied to the cells. This workaround will help you to maintain the visibility of the grid lines in your Excel worksheet.