formula changing text colour of different cells, effect another cell's value.

No. There are no formulas or settings that will change the color or value of a cell based on manually chosen colors in other cells. It is not possible for the value in a cell that was manually typed in to change unless something new is typed or pasted into it. The exception to this is an Applescript.

If the cell A2 has a formula in it, though, it can be 100% under one set of conditions (when A1=20% and B1=30%) and 30% under another set of conditions. The color of the cell can change based on whether it is 100% or 30% by using conditional highlighting. Still, the color of other cells cannot be used as a condition for conditional highlighting.

As BadUnit stated, there is no automatic conditional formatting that will do this - you can set formatting based on a cell's value, but not on its attributes - but it can be done via AppleScript.

For example, this script does what you ask - but you need to invoke it manually (it won't auto-update based on the changing values of the cells). Just past it into a new Script Editor document and run it. It should be fairly self explanatory, but post back if you need more help.

set blackColor to {0, 0, 0}
set whiteColor to {65528, 65533, 65524} -- I have no idea why Numbers uses this for White, but...

tell application "Numbers"
	tell table 1 of sheet 1 of document 1
		set a1 to cell "A1"
		set b1 to cell "B1"
		
		if text color of a1 is blackColor and text color of b1 is whiteColor then
			set value of cell "A2" to (get value of a1)
			
		else if text color of b1 is blackColor and text color of a1 is whiteColor then
			set value of cell "A2" to (get value of b1)
		end if
	end tell
end tell