Hi AllanR, thanks for helping me out.
I've added the gold projectile as a new animation and the original issue is fixed, but I can't work out how to get the enemy sprite to display the new colour, I just keep getting the original red one. I've tried adding a new event, sub-event and condition but none of these are working.
I also noticed the bug where the game doesn't end correctly sometimes. So far I haven't been able to work out what's causing it.