I'm surprised that they don't appear as characters in rescue bots

 

Except that they are both Yellow Kid Appeal characters in a Transformers series.

But You're right, Hot Shot draws much of his character from Hot Rod, not Bumblebee.

 

From what I understand, Bumblebee is supposed to make a guest appear in Rescue Bots at some point, but I agree with the Spongebob/Pikachu relativity theory. Yellow may be a caution or anger color, but as a future parent to be, I will say that a lot of parenting books suggest the use of light colors for young children, yellow being one of those.

My biggest thing about marketing a color like red or yellow is that it's a pretty insidious mind trick. A lot of childrens companies like Hasbro, Mattel, etc, all have on staff a child psychology professional to examine trends. And check some of the cartoons that are produced. You'll find at least someone on a lot of rosters that is listed as a Child Development Consultant.

Now if you don't mind, I need to go buy another Bumblebee.

 

I read a while ago that yellow is the first color your brain recognizes. So it could be, at least in recent times, a matter of attracting the eyes of kids to a certain product on a toy shelf.