In Calc, whenever you use a formula that references a given cell more than once,
the cell colour is not remembered.

Steps:
Write the value "1" in cells A1 and A2.
Create a formula in cell A3 that reads "=A1+A2+A2".
Select cell A3, then click on the box where you normally input the text. The
formula will expand on the cell, showing each cell reference, e.g. A1, in a
different colour, that is accompanied by a rectangle highlighting that cell in
the same colour.

You will notice that A1 will be highlighted blue, A2 in magenta, but the cell
references on the A3 cell will be in blue, red and magenta respectively.
Calc should check whether a given cell has already been highlighted in the
current formula, before using a new colour to highlight it (thus causing the red
highlight to be overwritten by the magenta one).

This issue has been around since I can remember (at least since 2.0). This makes
it extremely difficult to debug complicated formulas, as you have tons of
colours that are not used and that do not correspond to any highlighted cell,
and conversely you can't "see" how many times and where a given cell of a given
colour is referenced in the formula (since every time it appears it's in a
different colour).

I can attach (can't figure out how) a very simple .ods file with the steps
above, and a small png picture with a much more complicated formula, which makes
the problem very clear, in case people consider it worthy.

Thank you.