I expected the McLaren to be largely orange, with white, then maybe a little bit of black.
I dont think its a terrible looking livery, but I am a little disappointed.
As to why teams change colours, sponsorship. In 09 that McLaren was powered by Mercedes (silver arrows), and their main sponsor was Vodafone (red). Happens all the time with McLaren, in the early 2000's it was silver and black with West sponsorship, and in the 80's/90's it was white and red because Marlboro. Before that it had a white Yardley livery, and you have to go back to the late 60's/1970 for a proper sponsor-less McLaren orange car.
Ferrari are Ferrari, they'll always be red, but McLaren have always rolled with the sponsorship and adjusted accordingly. Same with Renault/Lotus. Yellow and black is more historical (even further back, teams used to run colours based on nationality), but the iconic all black and gold came from John Player Special cigarettes, white and orange came from ING, Alonso's blue and yellow came from Mild Seven. Damn, cigarette companies gave us the best looking cars.