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Add additional costumes to the coin class

Click on the design button, then click on the sprite button. Finally, click on the coin class.

Pick additional costumes

In the sprite class properties, click the coin image with the pencil to see the image picker.

Click "New Costume"

Select a gem

Click on the new costume image, select "Items" and click the blue gem costume.

Add 2 additional costumes

Repeat the above step two more times to add two more colored gems. Your image picker should look like this:

Close the image picker

Click the 'X' button in the upper right corner to exit the image picker.