finally it worked.
but really strange i apply two conditions for frmatting
1.Expression :count(preceding-sibling::my:group14) mod 2 = 1 and i set shading as "Yellow"--it has not worked.
and the secod condition is
2.Expression :count(preceding-sibling::my:group14) mod 2 = 0 and i set shading as "Blue"--it has worked.
when i use any one of those(1 or 2) its not working.
only the combination(1 and 2) worked. very strange,is there any specific reason for this.
Thanks,
Devendra