Taking a full color screenshot from the commandline with ImageMagick

I am trying to automate website screenshots on my (headless) server as follows:

Start a virtual X server:

Xfvb :1 -pixdepths 16,24,32 -screen 1 1080x1440x24 -nolisten tcp

First I tried taking screenshots with cutycaps:

DISPLAY=:1 cutycapt --url=http://myurl --out=screenshot.png

This works with full colors, but the webkit engine is too old to support our CSS, so the web page does not display properly.

So I tried Firefox and ImageMagick:

nohup firefox http://myurl &
sleep 4
import -display :1 -window root screenshot.png

Sadly, the resulting image has only 16? 256? colors. Definitely not enough.

So I tried xwd:

xwd -root -display :1 -out screenshot.xwd

Still no luck, still just 16 colors. Any ideas?