Yellow sheets and sticky material

I confess that I write the titles of these posts on the fly when uploading pictures.  Often when I look at my list of posts to write I have no idea what the title is talking about.  But luckily I have the pictures to keep me in line.

Two men (not pictured, though I watched them work during recess duty) are pasting different colored sheets over the yellow material that they stuck up first.

I should probably find someone who is familiar with construction and tell me what is really happening. I'm quite sure I'm not using actual construction terms.