I am currently waiting for delivery of an android tablet and intend to buy Photoshop Touch for tablet. However I am unable to ascertain whether there is a color layer. I would normally create an initial image that is monochrome to develop the tonal relationships. Then I would normally open a color layer at the top and paint in the colors on the color layer in order to keep the tones consistant. You may want to look at the Bobby Chiu Tutorial on YouTube to see what I mean. Is there a color layer? If there isn't then is it possible to achieve the same result in another way.
You could create an empty layer located on top of the B&W one and simply paint on it.