Look you not missing the ";" in your code?
"backgroud-color:red;"
you have to put a ; in the end
style = If (GetContracts.List.Current.Contract.ATIVA = False ,"backgroud-color:red;",If(GetContracts.List.Current.Contract.ATIVA = True and GetContracts.List.Current.Contract.StandBy = True,"backgroud-color:yellow;",If(GetContracts.List.Current.Contract.ATIVA = True and GetContracts.List.Current.Contract.Billed = False ,"backgroud-color:green;","")))