Convert a color in an image to semi-transparent
convert input.png -fill white +opaque "gray" -alpha set -background none -channel alpha -evaluate multiply 0.5 +channel -resize 128x128 output.png
- Also resizes down to 128x128 px
convert input.png -fill white +opaque "gray" -alpha set -background none -channel alpha -evaluate multiply 0.5 +channel -resize 128x128 output.png