Is it possible to use font color or fill color in Excel as a cell reference?

No not with colour (color)

Sadly Excel is formula / text based.

You may consider - (advanced users) using lists and combo drop down boxes with the options of time of leave.

Not dynamically, no. There are a number of things you can, depending on your level of Excel knowledge.

The easiest recommendation I could make would be to make two rows (or columns, depending on the orientation of your sheet); one for "time worked", the other for "sick time" (note: you could go with even more columns to capture things like "vacation" or "jury duty")

Then, just enter time in the appropriate column, and use the sum feature on the whole row or column.

Yes the formula would be something like this:

=sum(c9+c12+c18) or you can click on the E that looks like the greek alpha E in your tool bar at the top of the page then hold down the ctrl key and click on the individual cells that you want to add.

good luck...