[pygame] Apply alpha to another image

["Stuart Axon" <stu.axon+pygame@xxxxxxxxx>]

Hello,
 I'm wondering the best way to apply alpha from one surface onto another -

I can get the alpha with
pygame.surfarray.array_alpha

But can't see a corresponding way to apply that to a surface without
affecting the rgb values.

Basically I have two images, one paletted, I manipulate the palette
then blit it onto a 32 bit surface.
The second image has the alpha, I want to apply this to the surface,
keeping the rgb values - basically a very quick way of colourising
things.