The mystery of inks

[David Glasgow]

I am struggling to understand and use inks (Rev 2.9)

I have no idea how these things work, but what I want to achieve is  
this:

I have a jpg image of a human in swim-wear.  The background was  
originally a separate layer in Photoshop, and is a sort of grey green  
swirlyness.  I mention this because it could be changed to something  
else if it would help achieve my goal.  I want to have a primary  
colour show through the human figure but not the background.  So by  
changing an underlying grc fill, the figure should switch from blue  
to yellow, to green, etc.  I don't mind the colour of the swimwear  
changing, but the skin should take as much of the underlying colour  
as possible.

I made a little test stack which looped through all the ink effects  
and applied them to my image which was overlying a grc filled yellow.

First thing is that I was surprised how many inks made no apparent  
difference to the image (7), made the image black (5), or disappear  
completely (7) - irrespective of whether areas of the image t overlay  
the grc or not.  Why is that?

Second thing is that three effects stood out as close to what I  
want.  They were BlendMultiply, BlendDarken and SrcAnd.  Are there  
any significant technical differences between these, or between their  
effects on Windows vs Mac? Anywhere I can read up in them?  I see  
that SrCAnd is described as a "legacy"...... some legacies are good :-))

Finally, if I did change the jpg background to be transparent, or  
solid colour, or black or something, is there an ink effect which  
would show solid colour through the human part only?  I did try to  
understand the Wikipedia article on alpha channels, but my mind went  
matte and dodged.  Or burned, I forget which.

Thanks,

David Glasgow