Hi there
I wonder if anyone can help me & guide me to a tutorial where I can colour an overlay that is originally black & white as I have downloaded a couple (grey scale I think they are called mmmm lol) & don't know how to colour them. 
Thanks in advance for anyone's help/kindness. 
Take care,
Julie
Marisa made a tutorial exactly for this!
You can find it here.
There is also a forum thread dedicated to tutorials here.
Another way to do it is to Control-click the layer. Selects only the colored pixels and not the transparent pixels.