Hello,
Unfortunately its a bit complicated and the color is in 2 locations due to how the color on shows on 1/3/ of the top row area. So in the index.php file you'll have to look for the below ID and you'll see its a gradient and the colors are set with PHP. This sets the color on the far left and far right.
#s5_top_row1_area2
Then for the actual custom position itself you should be able to override the color with the below. Just add this to your custom.css file:
#s5_top_row1_area1 .s5_toprow_right {
background:#ccc !important;}
For the second custom position at the bottom its the same except this is the ID you need to look for in the index.php file:
#s5_bottom_row1_area2
Then for the actual custom position itself you should be able to override the color with the below. Just add this to your custom.css file:
#s5_bottom_row1_area2 .s5_botrow_left {
background:#ccc !important;}