I would think PS would be a better tool for this, with its match color, replace color commands; though you could experiment with FW's change color filters...
This is a guess, but you might try this....
Take your pink color and get its hue and saturation. Then make a dark color of the same hue. Your light pink color will have a high brightness; find something that is the same, but with a low brightness.
Then, make a rectangle of this color over your color image and set its opacity down low. You might also experiment with blending modes, maybe multiply.
The reason I think this might work is your use of the term "colorwash." What you want is similar to taking a watercolor and brushing it over. Watercolor paints are richly colored, it's diluting them with water that turns them light. Same kind of idea. Start with a richly colored version of your pink, then "dilute" it by setting its opacity down.
If your ultimate goal is to get the image to the Internet then try this. I know it worked for me.
Get the dimensions of your image. Create a transparent image in Photoshop with the same dimensions and fill the layer with a darker than needed color. Save the image as a PSD. In Dreamweaver create a div containing the transparent image using the PSD file (DW will convert it to a PNG for you) and using CSS position it over your original image. Adjust the layer opacity in Photoshop to suit.
This way you will have a separate image you can reuse or repurpose in the future if needed.