Picture Mix

I borrowed my father in laws strobes today, so thought I would mess about and be a bit creative,
One thing that did throw me though was all the background colours I used the same #number on elements 11, on the 9 individual photos as a fill layer, but when joining them and using a brush with the same # colour to cover a few lines where the pictures didn't quite meet, the colour was a different shade? how can this be?

IMAGE LINK: http://www.flickr.com …emphotography/9​686581067/

Cool sets. I think I could get my daughter to pose for me if we were doing something like that!

As for the BG color differences, could it be a slight difference in WB, or maybe tint? You may want to post that part of the question, along with the images in the post processing section. IDK...