What does it mean when a page has a yellow hallo? It means that the page is in the PC browser cache and not on the server. There are many reasons why it may not be on the server. To remove it go to Local Changes
What does it mean when a page has a blue hallo? It means that the page source is hosted on another site.
Is there a limit on the size of a wiki page? Per discussion on Page Too Large there appears to be a 5mb limit on page size that can prevent pages from being uploaded, which I believe is involved in the forking process.