Let them choose the color
You have checked the fit, the limits and the practical details. There is still one decision worth leaving to the person who will ride it.

THEIR OWN WAY.
You have narrowed the scooters down to two suitable options. Both meet the checks you needed to make. You prefer the quieter color; your child points straight at the blue one.
It matches their pencil case. That is the whole explanation.
We think that is a reasonable place to stop asking for reasons.
The grown-up part of choosing a scooter already involves plenty: the right riding mode, age and weight limits, fit, setup, protective equipment, storage and cost. Those decisions need your attention. A favorite color can belong to the child.

There is a useful order to this. Make the practical shortlist first, then show the colors available within it. Asking a child to choose from the whole range, then explaining that their favorite belongs to the wrong model, creates a disappointment you could have avoided.
Keep the choice honest, too. If only one color is available within your budget, say so. If there are two, offer those two. There is no need to make the decision look bigger than it is.
Then give the answer a moment before making a case for your own preference. Adults can turn “Which one do you like?” into a surprisingly long sales pitch for the one they had in mind.
That does not mean children need a say in every purchase. It means that when we offer this particular choice, we should be willing to accept it. There is no wrong explanation among options you have already checked.

At Kick’n’roll, “Trusted by Parents. Picked by Kids.” is a useful way to think about that division. We need to give parents clear information to judge a ride. We also want there to be something a child chooses for themselves.
Sometimes it is blue because of a pencil case. Sometimes pink has simply been their favorite for as long as they can remember. You do not have to turn either answer into a lesson.
And if the blue one is a suitable choice, you can close the other tab.