was hoping there was a simpler way around it :-/
There may be, though I'm not personally familiar with it.
Try this method, which prevents both layers from becoming grayscale.
Open a copy of your color image in Photoshop and change it to "grayscale." Then save a high-quality copy and give it a new name, eg. image_bw.
Next, open a copy of your original color image. Next, open the grayscale copy, select and paste it onto the color copy. The grayscale copy should be on top. Erase the areas where you want colors to show through.
Although this tutorial isn't about your topic, it will give you some information about using layers and the eraser tool:
http://www.digicamhelp.com/how-to-create-depth-of-field-using-layers/shallow-dof-with-layers.htm