Cell shading continually darkens when you hold down Ctrl and repeatedly select the cell in Excel
Assume that you select multiple cells in a Microsoft Excel 2013 or Microsoft Excel 2016 worksheet. You mistakenly select some cells that you do not want to include in the selection. When you hold down Ctrl and then click those cells in order to remove them from the selection, the cell shading becomes darker, and the cells are not unselected. Additionally, the cell shading continues to darken if you repeatedly click the cells.
This behavior is by design because of a change in the Excel 2013 or Excel 2016 graphics engine. This change enables you to highlight overlapping selections. If you repeatedly select the same cells, the cell shading eventually becomes too dark, and you cannot see the text in the cell.
To work around the issue, cancel the selection of cells, and start the selection process again.