In one of the early episodes of The Simpsons however, one of the now recurring characters, Mr. Smithers, used to be browned skin with a light blue hair instead of yellow skin and grey hair. This is a onetime case where the animators accidently coloured him and made him into an African-American, a mistake they would later change in order to avoid controversy. Back then, The Simpsons have constraint budget and altering the colour of one character would be too expensive. Matt Groening would claim now that the yellow colour is just a smart design choice to catch our eye when we are flicking through the TV channels, but for all we know, it might just be a cost saving method that works and defined the animation now.