Conditional formatting ties a cell's appearance to a value. In a table or matrix you can change background or font color, draw data bars, or show icons. Rules compare values with boundaries; field value formatting reads a color returned by another column or measure. Use either approach when it makes a specific exception easier to find.
Check zero, negative values and blanks before you finish. A blank can mean there were no transactions, and treating it as zero can falsely label an unsold product as on target. Data bars need a zero baseline so losses extend in the opposite direction from gains. Color should support the number, not replace it: keep the value or a meaningful icon so the reader can still interpret the result without distinguishing red from green.