I've been pouring over this book called Where's Dennis? that catalogs much of Ketcham's pre-Dennis illustrations.
While he was known for his line work, I was enamored with his choice of colors as well.
Check out the yellow and green. Both are rich and colorful and yet are not garish. I also love the subdued, responsible use of pink on both the dog and the blanket:
And when I say responsible, I mean not like this:
Also note the use of lines pointing toward the man, the main focus:
The boy on the left's multiple legs is a hint at Ketcham's animation past.
For my money, Ketcham drew the most appealing kids ever.