While you can't change ProSelect's background color, you can use the Room View feature to display items on a plan colored background. Here are the steps for how to do this.

1. In Photoshop, create a new image 2000 pixels wide by 1500 pixels hight (4:3 ratio).
2. Fill the image with the color that you want.
3. Use Save for Web & Devices (under File menu) and save it as a .jpg file (medium compression) to your Desktop with a name like "White Wall.jpg"
4. In ProSelect, open the Setup Rooms window (under the Resources menu)
5. Click the Add button to add a new room
6. In the Select Resources window, click the Add new image button (top center) and select the image that you just saved on your Desktop.
7. After the image has been added to your Resources List, it should be already selected so just click the Select button (bottom right)
8. In the image preview (right hand side), drag the ruler so that it is almost all the way across the room and in the Enter Ruler Length box enter a number which is about 10%  larger than the maximum image size that you wish to display on the wall. eg If you wish to show a 40"x60" image with a 3" frame then the maximum overall size would be 46"x66" so set the ruler length to 73" (66x1.1).
9. You can repeat this for as many different room colors as you like.

To display an item on this wall in Working with Images or Working with Layouts, right-click the Select Size View icon (the ruler) and select your 'White Wall" room from the list. Use the Larger Size or Smaller Size buttons to adjust the size.

To display an item on the wall in Working with Rooms, in thumbnail view, double-click on the room in the Library Rooms section at the bottom of the screen. Then drag your image or layout into the room. Use the Larger Size or Smaller Size buttons to adjust the size.

Note: The maximum ruler size that you can use is around 265" (6730 mm) which should be large enough otherwise you may need to make the ruler size smaller to enter this value.