Think of an overlay like a multi-media project in the physical world. You have a textured background, say a canvas that you've glued some tissue paper to, and now you want to paint a wash of color over they top. Well in Photoshop, you can not only create that layer of color, you can even give it different effects! Let me show you...
HOW TO ADD COLOR TO YOUR JPG TEXTURED OVERLAY:
1. Open the textured overlay in Photoshop.
2. Create a new layer (Layer>New Fill Layer>Solid Color), click OK on the new Color Fill layer
3. Grab your eye dropper tool and pick a color from the Color Picker window. Experiment! You've now created a layer of color over the top of your textured file.
4. With the new color layer active, go to the Layers palette and the pull-down menu at the top where you can set the blending mode for the layer. This enables you to create different effects between the two layers. Experiment to see which layer effect you like. Try the 'overlay' and 'lighter color' layers. You can also adjust the transparency if the effect is too severe.
Here are the results using one of my textured overlays from my new 'Art House Textures Collection'.
For more detailed instructions, see my book 'Create Your own Printable Scrapbook Papers', found in my Etsy store HERE. You get 135 customizable vintage styled overlays in this book for personal and commercial use for just $13 - awesome value!