additional field validation

Hi,

for example - I have a table element and one of its coloums is of type TIME. The automatic field validation (only valid times allowed) is working fine, but now i need to check that only full and half hours are entered. So if the user enteres "11:11" in a row I want this input-cell to be displayed with a red border and a message to be displayed.

I can check and show a message, but how to highlight that special cell?

Can I assign additional value checks to a context node and how to do this?

Whats the best approach?

Thanks for any help,

Stefan.