I'd a yellow MINI One in my showroom last year with a sunroof, it took forever to sell it. They just look dated, but the toughest thing is keeping them clean. If you open the doors on any MINI, the inside is body colour, and dirt wells up in there in the tightest spots. This isn't an issue on the darker/more defined colours, but it stands out a mile on the Liquid Yellow.
If you were to go for the Liquid Yellow and couldn't talk your son out of it, I'd ask the Dealership where your nearest BMW approved body shop is and have the roof sprayed Black. It ads a very masculine touch to what could otherwise be considered a feminine colour scheme.
I'd still be inclined (If it were my choice) to opt for Pepper White, it's so stylish. If I was buying with my own money, it'd probably be a Pure Silver with a Black Roof (My own demonstrator at the mo has this spec), but the Pepper White would be a damn close compeditor.
At the end of the day it's your money I guess, you should buy what keeps you happy. Teenagers are extremely fickle, and even though the Yellow is the best thing ever at the moment, next week it may not be
Owen.