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I once read somewhere that there was a hack or patch for Battle Garegga where they changed the grey/brown bullets to a brighter color.
A.) Is this possible to run this Rom hack/patch on Mister?
B.) If So, Could you explain it to me like I'm 5 
?
I want to enjoy this game, its just that my sight is not so sharp and a brighter bullet would be much easier to avoid.
I also had trouble with the "long" bullets in Garegga, my sight can't differentiate them well from the background and all the debris. There is a type 2 veersion (not a hack) that changes them to round bullets. In Bakraid, you can even change the colors for the bullets in game.
I explained this here:
If you have any question let me know.
Battle Garrega Shottriggers for PS4 has multiple different options for changing the bullet color but I don't believe there are any hacks for the arcade rom.
The Garegga Type 2 rom does that, but with no options, some bullets just have different colors.
In Bakraid, you can choose the colors of the bullet using an in-game combination.