OK, I have figured a way to save the background color. It's pretty bad, but it does what it does.
On the save button click but before the save file dialog, do this:
telep.SelectionBackColor = telep.BackColor;
then on the load button click, and after the load file dialog, do this:
telep.BackColor = telep.SelectionBackColor;
All this does is it highlights the text in the same color as the rich textbox then saves. And After loading, It changes the Rich text box back color to the one of the highlighted text.