Conditional formatting based on other cell value

what would the rule be to change the font colour

you make the font white - which is the same as the default fill colour and so becomes invisible

But conditional formatting will need a rule to apply to the cell , why should the font tune white ?

Conditional formatting is one way to go, the other is to use the same condition to display a null string.

So if you want the cell to go away when G6 > 100 and display G1 + G2 otherwise, then If (G6 > 100,"",G1+G2).

Since you want the computation always to be done, you certainly need to use conditional formatting, not make the cell blank.

An example:
Column C contains a sum of columns A and B, but when the number in column A is >100, you don't want to see the result in C.
Select the range in column C (C1:C100 for example) then click Conditional Formatting, 'New Rule' then 'Use a formula to determine which cells to format'.
Put this in the formula box:
=A1>100
Then click 'Format' and select the 'Number' tab. Click 'Custom' and enter this custom format:
;;;
That's three semi-colons. A custom number format works like this: positive format ; negative format ; zero format ; text format. So ;;; makes the cell display absolutely nothing - but this will only happen if column A is >100. The number is still there - you can still do calculations with it - you just won't see it.

Hope that helps.