In the bottom section, blocks will only stack if they are the same colour. The red block in the diagram will NOT stack onto the blue block because it is a different colour. 
If you try to stack a third block it will clear the stack below it. In the diagram, if the blue block is not caught it will clear the two blocks below it. This is handy if you need to remove an unwanted colour but bad if its the colour you are trying to complete. 
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