According to the SCMP article, the paint was actually more expensive than the food coloring. The staff used paint allegedly because they wanted the colors to be brighter. In fact, most of the school's staff consumed the lead-tainted food just like the students did.
So there was no financial motive or "corruption" at the school. It was just plain stupidity.
The main issue isn’t so much that the staff is stupid but that local government’s first gut reaction is trying to cover the whole thing up.