Email doesnt support background colors any longer. To get around this you would need to create a cell and use that cell to control the background color of that cell.
To do this go to your toolbar and create a table. If only one cell then you would choose 1 row by 1 column. Then make this the size fo your emailer. Place all your content within this cell. Finally right click on your cell and choose properties. Within here you can set the background color.
When done choose ok and dont forget to save your work.