For this tutorial, I am using PSE9 on a Mac, and I'm using my free 3x4 overlays which you can get on my Facebook page.
I'm going to show you how I recolored the "brand new" overlay.
First, open up both your image and the overlay file in PSE.
On your image file, crop to the size of your overlay. In this case, 3x4.
Once you've cropped your image, head back over to your overlay file, and hold the command key and left click on the layer thumbnail. This will give you marching ants around the overlay.
Go to the edit drop down, and fill selection.
You'll choose the color you want the overlay to be, in this case, white.
Hit ok, and the overlay will become your chosen color. Make sure you deselect everything.
Now click on the move tool, and drag you overlay onto your image file.
I like to change the opacity of the overlay so that it's not so strong, but you can leave it at 100%, whatever you prefer.
Then flatten your image and save.
I hope some of you newbies to Photoshop Elements and using overlays find this helpful!