Changing TEXTBOX color changing when we enter a value lessthan 28 in asp using C#

Question

hi frnds ,

 please help me in changing TEXTBOX color changing when we enter a value lessthan 28.

  if value entered >=28 (greater than or equal to 28) then no need of color change in TEXTBOX.


  if value entered < 28(lessthan 28) then textbox color must change to red


thanks in advance....

chiranjeevi