During my time at ABCmouse/Age of Learning, I worked as an illustrator and designer for online and printable activities.
One series of projects, paint-bys, were designed to be based around core concepts & characters introduced in the lesson. With multiple challenge levels available to children, they would color sections of the illustration using the 'solution' of a problem's corresponding color in the color key. This would then reveal the completed illustration in its entirety.
Each 'level' of the paint-by would offer more complexity in the design, attempting to break of the main image so that it would now be immediately apparent what the completed image was.
Sometimes grade level, the completed scene desired, or other factors would also play a role in how complex harder levels could be.
Once completed, the illustration would be apparent to the child, and they would have had fun exercising a lesson in math, reading, or other critical subjects.
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