human biology - Can someone explain the color-changing unit (CCU) to me?

I've been physically carrying out serial tenfold dilutions on samples of Ureaplasma to work out the color-changing units (CCU).

As a definition, the CCU is the highest dilution at which there is a color change.

If the highest dilution is 10^3, apparently that means that there are 10^3 cells per ml in the original sample. But this is what I do not understand. How are we sure of this?

I feel like there is a lot that I am missing.

Any help would be greatly appreciated.