So prepare ahead of time and make sure that you've got enough distractions for the kids so that the trip will be uneventful in terms of fighting. Siblings don't usually like sharing stuff so you'll need at least two of everything. Two sets of crayons, two sets of coloring books. And it's probably best to make sure that both sets are exactly the same so that they don't get into an argument over the perception that Billy's coloring book is better than Casey's.
The first coloring book, "The Little Folks' Paint Book" was published in 1879. Crayola introduced the crayon in 1903. And the average American child spends 28 minutes a day coloring and wears down about 730 crayons by the age of 10. Between parents and schools, roughly 2.5 billion crayons are purchased each year.
Learning the color of a chair, or an elephant, of flowers, sun, zebra, parrot etc, will not need mugging up, but mere practical activity that will leave an everlasting impression on these little developing minds. You can also engage them in activities like creating new colors by mixing 2 or 3 colors. This will make them understand the evolution of different colors, which is fun too..