VBA to sum cells by background colour

Hi
Can someone help me with a bit of VBA code please as I can't get a formula to do what I need.
I am trying to sum columns which have the background colour manually set to red or green.
They track invoices awaiting payment in red and those paid in green and I change the colour
when an invoice is paid. The code needs to sum the values in the red cells only, in order to
show the total amount unpaid and automatically update the total when a cell colour is changed.
There are 12 columns, one for each month so the code needs to work on each column independently.
I'm using Excel 2010.

Hi KOKOSEK
Thanks for your reply. I have tried your code which runs but I can't see any result in cell A1 when running it.
However it gives a total amount across the full range and what I would like is a total for each column. I have
attached a sheet showing how I would like it to look.

The code is the same with KOKOSEK.
But this one is added with looping for 12 times (for the months),
and the code runs after activating Sheet1.

I added one more Sheet, which is Sheet2.
When you open the attached file, it will be Sheet1 and there's no SUM in A1 to L1 for the red cells yet.
You need to go to Sheet2, then go back to Sheet1.
After Sheet1 is activated, the code runs then show the SUM of the red cells in A1 to L1.

Since I don't know how is your list is going look like,
I just put 25 times looping to check each row in each column.

KOKOSEK and karmapala

I have edited the code and put it into the actual workbook and it does exactly what I need.
Many thanks to both of you and I will mark the thread as solved.

glad to help gasdesign