There is no way to change the color of a particular cell.Tableau is not like excel. If you have any particular scenario then please post the workbook .We will help you.
Check out this thread, there are ways you can color specific cells, it's just not as straight forward as Excel is in the surface.
Hope this helps!
Please consider posting a sample TWBX/packaged workbook with a mocked up scenario and include what you'd expect as sample output.
Pretty much all you need to know about Conditional Formatting in Tableau can be learned from this link, but a warning: its time consuming and a bit difficult to maintain a view using the Multiple Axis Crosstab. You can download the TWBX from this link, and read/study the captions to learn all about the options you have inside Tableau.
As mentioned previously, Tableau is very different from Excel, and is tied to the underlying records/rows in your dataset. When measures exist on the same row, it is difficult to isolate them based on how Tableau works.
Cheers and best of luck.