First, select your list. Right click and choose Filter | Filter by Selected Cells Color. You can then select the filter drop down in your list’s first cell and choose which color to filter. Next, in a row that is outside your list, type in the following formula: =SUBTOTAL(103,A1:A100), where A1:A100 are the cells in your list. Whenever you filter the list, this formula will tell you how many cells are not hidden.

You can remove the filter at anytime, by choosing the Data tab, and clicking the Filter button, and the formula will give you the count of your whole list (i.e. 100 in the example of A1:A100).