So what you want to achieve is if Field1 is greater than Field2 then the background of field1 to be red? If that's correct then copy your expression from Expression atb (from test) click the + sign and copy the expression you have there. Now go to Dimensions tab click + sign for FIELD1 click Background Color and click on Edit then paste your copied expression there. Finally click Ok. That should be it
Must you concatenate Field1 and Field2? Can you not leave it separate? This will make it easier to highlight whichever is greater between both field. And if you must concatenate both fields then here's my suggestion.
Have 1 more expression using
=if(FIELD1 > FIELD2, FIELD1 ,FIELD2) // in the expression tab
Label it as Test2
Then in the Text Color (click + sign from Test2 ) and enter