Probably something like below:
if(less(item()?['Net'],0)),
concat('<span style="color:red">', if(empty(item()?['Net']),'$0.00',formatNumber(float(item()?['Net']),'C2')), '</span>'),
if(empty(item()?['Net']),'$0.00',formatNumber(float(item()?['Net']),'C2'))
)
+ the replace(...) in the email body