Avoid large solid areas of all "pure" colours: red, blue, green, magenta, cyan, yellow and white.

Also, as a side note, any areas of black you may have in a glow sprite must be masked out. What's more, to avoid odd drawing errors, try to avoid glows that have areas that are even close to black. When making my glows, I always cropped out any values within a certain value space of black.

all the best,

Dave @ ATMOS