I have an image and I want to make it lighter at the top fading to no effect at the bottom. So if the image was a solid green to begin with, at the end it would look like a gradient from light green to green.
I thought I could do something like this (copmositing a white block over the top of the image using a gradient as a mask):
convert a.jpg \( +clone -fill white \) \ \( +clone gradient:white-black \) \ -composite out.jpg
Or this ('colorize' an image with a gradient that fades to transparent)
convert a.jpg \( +clone 'gradient:black-rgba(0,0,0,0)' \) \ -colorize 50% out.jpg
…but neither works.