You have to make all the coloured lights around the outside white.
I think it is easier if you start with the corners first.
If the light in the top left hand corner is RED, Then in the top left hand square put a BLUE arrow pointing towards the corner & the red light will turn purple.
Then diagonally behind the BLUE arrow, put a GREEN arrow facing the same corner & it will turn WHITE.
Then move on to the next corner. If it is say CYAN (green & blue) then just point a RED arrow at it & it will turn WHITE.
It doesn't matter if the arrow is 2 or 3 squares away as long as it points to the light you want to change.
You may run out of squares where you need them, in which case you just have to reset them & start again (I forget how you do that, but there is probably a lever )
The starting colours are random, if your lucky you will get a lot of cyan, yellow & purple lights to start with. (only needing 1 arrow to turn them white)
Or better still, a few white ones.