OK I have everything figured out now. 
For my Sheet2 I am using the code below.
It tests the current cell's font index value.
If it is automatic, it changes the colour to black and then to magenta.
If it is black, it changes the colour to magenta.
If it is magenta, it changes the colour to black.
This is basically a double-click toggle between two colours, which is what I was looking for.
For my Sheet1 where I want to double-click toggle between two colours for an entire row, except for one column, I change Target.Font.ColorIndex to Target.EntireRow.Font.ColorIndex.
As for that one column where the font is either green or red, it isn't really a problem and I don't need special code for it.
This is because cells in that column are getting their colour from an assignment I have made outside VBA. If value <0 red, if value >=0 green.
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