I don't understand those kids toys where you can write on the walls and then wash it off. Like those Bath Tub Writers. Or that mess free finger paint. It tells kids that it's ok to draw on your walls and furniture. Either it disappears or you just wash it off, no big deal... Yeah, I'm not a fan. I think that kids need to learn responsibility and they need to have consistency in order to connect the dots. You can't set a double standard. "Well, if you use these special crayons you can color wherever you want, but if you use any other kind of crayon you will get in trouble doing what I said you could do two days ago" I think that is cruel to do to the kids and it just makes for more trouble for mom or dad later.
Now, I don't however have a problem with those aquadoodle things. The pens you fill with water and write with them. My daughter has one and she knows it's just water. She also likes to squeeze it and squirt it all over the place. But our rule is that when she runs out of water she's done. Of course, she's only 2, so she doesn't really get that yet, but she will only learn if we make it a standard now. The difference is that it's not actually paint or ink. She knows it's just water and she is able to make that distinction between a marker and water. And when she's out of water she asks "More water please." HA! of course I have to say no.
One of the first "painting" toys my daughter had was a bucket and a paint brush that Grandma let her use on the back deck. When it's really hot outside she can "paint" all she wants and it dries right up. She doesn't even realize it's gone, I think, she just keeps having more space to "paint" but again, she asks for "More water please." She knows that it's just water, the same that is in her pool.