I'll let the SC answer your questions, but if it's allowed don't go too much into the digital art realm or your score might suffer. I enjoy digital art but others may not score kindly.
I've read the rules .. I don't see anything about using in program tools like the paint brush on a new layer.. or anything about using layer modes for that matter.
"You may not post-process your entry from or to include elements of multiple images, multiple exposures, clip art, computer-rendered images, or elements from other photographs (even those taken during the challenge week), and other similar items. "
Can one of the site council members please clear this up?
My thoughts on using the paint brush. It is somewhat answered in the rules under the part about computer rendered. You will be using the paint brush to add an element that is not a part of the original picture? If it's enhancing something there in the picture as it was taken on the camera, then it would be legal, if it's making something that isn't part of the original, then it is computer rendered.
Just my thoughts. Hope you get a good answer soon from the sc.