If you want to highlight the negative numbers in red in a Google Sheets document, then there are two ways for doing this. You can use the conditional formatting or custom formatting option present under the Format menu of your Google Sheets document. While the first option lets you fill or highlight the selected cells (having negative numbers) with red color or a custom color, the latter option lets you highlight negative values in selected cells with red color only. We have already covered both the options separately with a step-by-step guide in this post above.