I have completed the task, all of your request, I can send it to you anytime, just hire me :D
My implementation will only add 3.95KB to your server, 3 files:
1. [login to view URL] (this is the minified version of the javascript that I wrote for your request, but of course I will give you the unminified version with my comments)
2. [login to view URL] (background for the first date selected, upper triangle green, lower yellow)
3. [login to view URL] (background for the second date selected, upper triangle yellow, lower green)
I decided to use yellow for the selected dates, but of course if you want me to change it, it,s really easy to do so, you can even change it yourself :D just take a look at the source.
Its really simple to implement this on your website:
just add <script type="text/javascript" src="[login to view URL]"></script> anywhere with in the document as long as it's after the jquery script.
for the image files, I placed it at the root, just like your [login to view URL]
I also took a video for your reference:
[login to view URL]
though after the point the video was taken, I was still able to improve it more :D