Concept and visuals are simple and fit well together - lets the player focus on the gameplay, especially as it ramps up. A suggestions -would be nice to add a buffer period between category switching. It can often happen that the items will drop in but the category will switch right before scoring and may count the item as wrong (e.g.. a red triangle 1 will drop into the 1 slot as soon as it changes to shapes and the triangle will no longer fit the circle that it turns into).