I think people used to keep it a secret so that other girls wouldn't wear the same colour as the bride! I know it sounds really stoopid but girls can get so stupidly jealous and competitive over silly things! Even Aunties! Lol!
I didn't see the big deal in keeping it a secret and when anyone asked, I told them openly. Big mistake! Lol! So many girls and aunties wore the same colour as me! At first I thought it was just a coincidence and then I realized it wasn't! Ha ha! I just laughed it off but later on I did wonder why they did that?!
But then on one of my cousins shaadi, everyone found out she was wearing a tea-pink colour. My other cousins intentionally wore pink. One of my Aunties was advising me to wear pink b/c the Bride was going to wear pink. I was thinking WTF? In one of the wedding photos I took when all the 'pink ladies' went to the stage to take pics with the Bride, the Bride completely faded in with them even though she was looking stunning.
But I think some brides keep it a secret simply to give a nice surprise to their family and guests.