In Microsoft Excel, if a cell contains a formula that results in an error, a triangle appears in the top-left corner of the cell. The triangle is an error indicator.

How to Make Colors Change With Validation in Excel. Excel's conditional formatting tool applies formatting depending on cells' contents. Validation rules can change cells font, style, size or border.

To change Gridline color in Excel, use Excel Options, apply a custom-colored border to the cells, or apply a custom line color. Full details here.

The default active cell format in Microsoft Excel isn't all that easy to see, but you can use this easy-to-implement code solution to change that.