Creates a 10x10 px canvas of noise, chooses a random pixel from it, creates a 500x500 px canvas of that colour. The use of random on an essentially random image (noise). is probably overkill, though I suspect neither algorithm is truly “random” so the additional code won’t hurt

  
#!/bin/sh
px=`jot -r 1 1 10`
py=`jot -r 1 1 10`
convert -size 10x10 xc: +noise Random seed.png
convert seed.png -fx 'p{'$px','$py'}' -scale 5000% colorChoice.png
rm seed.png

– James