selection screen Field gray

Hi All,

I want to gray the two fields WERKS and BURKS.Here is my piece of code.

selection-screen begin of block prog with frame title text-001.

parameters: p_werks type pa0001-werks default 'ABC',

p_burks type zabc-burks default 'Y'.

selection-screen end of block prog.

initialization.

if p_werks = '1221'.

loop at screen.

if screen-name = 'p_werks'.

screen-input = '0'.

modify screen.

endif.

endloop.

endif.

loop at screen.

if screen-name = 'p_burks'.

screen-input = '0'.

modify screen.

endif.

endloop.

Please help me in fixing this issue.