Indeed there are two reasons. First is visibility. Cartoon characters color schemes are based on making them stand out from their backgrounds. Since the sky is background so often, the color diametrically opposed on the color wheel, yellow, is used.
The second reason is psychological. The color yellow is associated with happiness and fun.
Now, as bridge players, we could wear yellow to make everyone around us happy. OR!! We can make our bids bright and clear so they stand out. Let's try that today.
