Mix the flour and salt.
Add in the water until the dough is moistened but not sticky. If the dough is sticky, add a bit more flour to get it right. I use my stand mixer with the dough hook attachment to avoid all the stirring for this step.
I always have to go back and forth between adding flour and adding water until I get the right consistency. Since you're going to add powdery paint, it can be a little moist for now.
Separate the dough into 4 fist-sized balls. Return the first small ball of dough to the mixer and pour in the powdered paint. Use the dough hook to knead the color into the dough.
Play with the dough. If you mix the colors right before rolling it out, you'll have a pretty marbled dough similar to the first photo below. If you mix the colors and then play with it a while, you'll have a blended color like the later photos show.
To make shapes, sprinkle work surface and rolling pin with flour. Roll out the dough to ¼"-½" and use cookie cutters to cut the shapes.
Bake the dough in the oven at 250ºF for 90 minutes, until it is dried out but not browned.