You can view the mask of a layer overlaid on top of the canvas by pressing \ (backslash, usually the key above return). This toggles it on and off for the currently selected layer. You can also open the channels panel and toggle it from there.
Be careful though. The small box around the layer contents and mask indicates which one you're painting on.
Here's what it looks like when you're painting on the layer itself.
And when you're painting on the mask.
You can change the overlay colour by double clicking the quick mask button in the tool panel (second from the bottom).
Or by right clicking on the mask in the layers panel and choosing Mask Options.
Photoshop CS6 shown, but this stuff hasn't changed for a while. Should be almost identical in all recent versions.