how to automatically activate a macro when the font color in a cell changes to black

As I would appreciate any help, let me elaborate on the programme. I have a summary sheet in which I input readings of survey instruments every week or every month, say. For each new reading of a survey instrument, the reading which I put in the summary sheet is then captured and displayed (via formula) onto a detailed table of readings (on a separate worksheet) of that instrument. Each row of the detailed table shows the reading for a certain week. Once a certain week's reading has been shown in a row of the detailed table, I will then change the reading (computed via formula) to its value. Any new reading for the instrument that is manually typed in the summary sheet will then be captured and shown in the proceeding row. If I do not change the computed result/reading (formula) to its value, then any new reading will still be displayed in the same row as the previous week reading, that is the new week's reading will overwrite the previous week reading. I wish for the changing of the formulas to its values to be done automatically instead of my current method of using a shortcut key to run the macro to change the formula to its value.