You have to color the white climbing wall blue, thanks to which you will be able to use it.
The mission is neither long nor difficult. First of all, drive southwards, where there is the mission marker on a rooftop. To climb up to the roof, you first have to paint a fragment of the white climbing wall blue. Colors are available behind the shopping window of the next building. You will enter there using the astronaut crate at the entrance, which can be rebuilt into a teleporting device. Use the first color swapper on the left. Then, climb up to the rooftop and start the mission.
You should have no problems with this part of the mission. You have to follow a specific path to the the rooftop of one of the buildings. You can eliminate the few opponents in your path by throwing them off the building. At the end of the path, you will reach Blackwell and you will face a larger group of opponents. Once you defeat them. Escort the man back to the destination, on the vehicle.