[web2py] flash obtained during not entering a value

[dantuluri jaganadha raju]

Hi,
                    I am getting "enter a value"(with red colored back 
ground) when not entering any data to field of table (to which i made 
constraint IS_NOT_NULL). How can I change that flash to some "DESIRED 
ONE....."(including back ground color) for  'ONLY SOME'  fields of table


                                  Thank you in advance...

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