Do you mean the colour of the page overall or just the bit behind the post?
The gray bit is an image called kubrickbgcolor.jpg and basically what you need to do is create an image in the colour you want (just a small one say 100 by 100 pixels) and save it with that file name. Then you need to use FTP (you’ll need a client like smart ftp) to upload it into the images folder within kubrick. The process is the same to change the header image, footer, colour behind the posts you just need to change the file names. You can use any image programme as long as you save them as .jpg