auto/conditional formatting
 

have the following sheet:

Column A = value inserted by user
Column B = value inserted by user
Column C uses index function using column A + B to return a formula
The next 5 columns (D-H) will have additional values, when filling out the
form these cells will be empty and will be inserted by the user.

My goal is that excel automatically gives the cell a shade/color/marking etc
when a value needs to be added.

Example:

Column C

=(D12+E12)/3 * F12 -- I want Excel to shade cells D12,E12 and F12

=(D13*3.14)/G13 *H13 -- I want Excel to shade cells D13,G13 and H13

Now it is easy to see which cells require a value to be inserted by the user
or when a value is missing.

Is this possible with conditional formatting using a formula? Or do I need
to use VBA :-( (dont have any knowledge of VBA). It should be possible; I
mean the trace precedents tool shows you all cells used in a formula/cell.

Thanks,

Willem