If you are using data validation for validating the user input you could use Circle Invalid Data which is part of the Data Validation feature. Here you can read more about how to Circle Invalid Data
If you want to set background on a cell you could set the Fill of this cell to red. Please note that if you set the fill of the cell this change will be included in your document on export. Here you can find how to set Fill to a cell
If you do not want to use Circle Invalid Data and you do not want to change the document but simply show a colored rectangle behind the cell values, you could implement custom UI layer. This forum post shows how to implement a custom UI layer
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