4 different random objects with no duplicates

I'm looking to have 4 different objects (in this case it would be 4 different colours)spawn at 4 set points on the screen (the four corners). Every X seconds though the colours in the corner would randomly change.

An example:

Corner 1: blue

corner 2: Green

Corner 3: Red

Corner 4: Yellow

------------------ 2 seconds come up

Corner 1: Green

corner 2: Blue

Corner 3: Red

Corner 4: Yellow

The problem i'm having is I keep getting duplicates of the same object. I've tried various different methods of trying to only have only colour up at a time, have messed around with using different animation frames as opposed to objects, but still not having much luck.

I'm pretty sure I'd need to use animation, as I'd want to anchor these four points to the screen to avoid any bugs with different screen sizes

Thanks for reading :)

Here's one way: randomColors.capx (r113)

This is exactly what I was looking for, thank you soo much :). Will put you in the credits and once again big thank you :) :)