about get max value

Hi,

I want to know how to sign a summary field with some colors.

For example,I made a report about every itemgroup's sales in each client.So I made two groups,the first is client,and the other is itemgroup,the sales is the summary field,I want to format the max sales values with red colors in the every client group,but the function maxium can not support the summary field,how can I do? Thanks.