Show the standard error and show the Input field in red

My web dynpro has a input field. This input field is mapped with a context attribute which type is I.

I want that this field doesn't show the default value 0.

If I change the attribute context to String I lose the standard message "The Entered Value Can Only Contain Numerical Characters" and the input field is not in red with this error.

1.- If I use a context attribute type I, How can I show an empty default value ?

2.- If I use a context attribute type string, How can I use the standard error "The Entered Value Can Only Contain Numerical Characters" and show the Input field in red?

How do I solve this?

Thanks