Below is the final answer:
Explanation:
I think the main question is, which color will go through the center column?
There are 4 different colors, so there are 4 possibilities. The main reason you'll want to use this gap, is that another color could go around this color. This gets the second color 'at the other side'
Note that both B and C are adjacent to the cells connecting the center column gaps.
So if B or C is using this center gap, you can't use the importance of this center gap.
The next thing to notice is the combination A - B - A at the right of the puzzle. This B is trapped between the 2 A's, So something has to happen if B wants to escape to the other B. On the other hand, the one D is completely free, so the D probably doesn't have to use the center gap. If now we know the A is going through the gap, we would have the following. (let's immediately avoid the D's, to make it easier to connect the D's later on)
This now forces the below C to the right, and that makes there's only one way on how the A is going through the gap:
And this let us see how the B's and the D's should escape, and the final answer reveals us: