Count coloured cells

I have this spreadsheet that uses colour fills on some cells to indicate when a batch is made available late, early or on time. This information is not in the sheet but known by my colleague entering the information, so there is no conditional formatting involved. So my colleage adds the data, and then formats the cell colour as appropriate.

At the moment our figures for "total made available late" are added manually. I am hoping to use a formula to sum the number of cells that are red, or yellow, or any other specified colour.

The problem I have is that I do not know how to do this, and a quick internet search has not helped much. I understand I might be able to use Cell("color",[reference]), but I am having trouble understanding how this formula works.

Any advice?

That works perfectly, thanks.