So this might seem like a stupid question, but let me explain

At the moment I'm playing a family with a houseboat. Obviously there's not enough space to put a parking space there, so the only option for me to buy a car is to click on a parking space in town and buy one. But I can't change the color there. I can't even choose one of the premade ones you usually find in Buy Mode without using CAS tool.
Is there a cheat or something that lets me change the color?

I started to suggest having your family buy a small, empty lot in the world and then buy the car on the lot when the full buy mode is opened with the Real Estate options, but then I remembered testingcheatsenabled true also lets you right-click or shift-click (I forgot which) on a lot and "Buy on this Lot." (Along with Teleport here and Build on this lot.) Once the full buy mode is open you should be able to get the car the color you wanted.

At first your tip sounded great, but I had to play around a bit. It seems like it only works if I own that lot. So I bought a tiny empty residential lot (cost ~ 1.000 Simoleons), put a parking space there and bought a car by clicking on that parking lot. Then I pulled the car out of of my inventory, put it down at that parking space, switched colors and put it back in my inventory. And then I sold the lot again.

Seems like a huge effort to simply change colors, but at least we found a way to make it work Thanks!

The move objects on cheat will allow you to place the car or boat parking spot anywhere on your lot. Change the color, delete the space, turn off the cheat. Voila!

testingcheatsenabled true
Hold Shift + left Click Mouse on the floor or ground of lot you want to edit.
Buy on this lot
This allows me to change things on any lot in game w/o having to go into Edit Town.
If you do purchase things for the lot it will come out of your household bank account, but if you are only editing the colors it shouldn't make a difference.

When space is very limited, I just use the 'moveobjects on' cheat and place the vehicle without using a parking space. Recolor, turn off the cheat, then move the vehicle to my Sim's personal inventory.