Pseudo Transparant images

Question

Hello World!

For an assignment at university we program in a pretty unknown language Modula 2, which lacks major graphic support.

I was wondering how to achieve a 'transparency' effect on images, i figured it would work like this:

Create a 2D array for the background area of the image filled with the colours of the different pixels in that area, create another 2D array of the image with again the colours of every picture and than merge the pixel colours and draw the different "new colours" on their appropriate place.

What i was wondering about: how do i merge the colours (hexadecimals) just:

( colour1 + colour2 ) / 2 

?

Thanks for your help!!