Today I needed to convert a transparent Grayscale PNG image to the same format, but in the RGB color space. (some processing software that we use cannot handle grayscale transparent PNGs for some reason.)
Ultimately this is what worked for me. I was only able to figure this out through several hours of experimenting with ImageMagick options, so I hope this will help someone.
convert in.png -type TrueColorMatte -define png:color-type=6 out.png