Ah. I'm keeping the white, then! 
I tend to forget to pick colours when using paint since I'm so used to just right clicking in gale... "orz
I didn't know of this tool, thank you so much! :D 
File updated, it now has 12 colours. 



 

Since you have the red glow fading into the white, I think it looks better with a white background than a transparent one :]

You can always check how many colours your work has using the PJ Image Inspector.
Looks like this has 21 colours. Some of them are redundant xP When pixelling, I think having a habit of sampling colours from your image as much as possible instead of using the swatches/colour picker helps a lot with avoiding useless colours.