applying a colorfill to a bitmap?

possible to do it with .net? or do I have to do it pixel by pixel?

here's an example"
http://www.vbforums.com/attachment.php?postid=1410508

and here's a color fill applied, with the blue color
http://www.vbforums.com/attachment.php?postid=1410509

ignore this

I can easily do it if the bitmap doesnt include any transparent pixels, but if it does, then the colorfill is applied to those pixels as well. What I'm doing is that I'm creating a semi transparent brush and I'm painting a rectangle over the picture. But how would I aviod the transparent pixels?