Is there a way to insert a formula that will add cells that are formatted using a specific fill colour, eg yellow?

No, you have to use VBA and it gets very, very messy if you use conditional formatting to color cells.
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Jim Cone
Portland, Oregon USA
XL Companion Excel add-in... for the business analyst

Hi,

Follow the steps below:

Select a cell for example A1, then go to Formulas tab -> Defined Names group.

In the New Name dialog box, type "IsFormula" in Name part, type "=GET.CELL(48,A1)" in Refers To: part. Then click OK.

Then select the range you want to highlight the cells contain formulas for example range A1:C10.

Then go to Home tab -> Conditional Formatting -> New Rule...

Then enter as the following picture.