With this code:

      !/usr/bin/ruby

      require 'RMagick'
      include Magick

      imagelist=ImageList.new('/home/jan/Desktop/test.jpg')

      collage = imagelist.montage do
      self.border_color='red' self.border_width=10 self.geometry=Geometry.new(500, 500) end

      collage.write('/home/jan/Desktop/test_collage.jpg')

      IMGCOMMAND="-border 10x10 -bordercolor red -geometry 500x500
/home/jan/Desktop/test_collage_original.jpg"
      system "montage /home/jan/Desktop/test.jpg " + IMGCOMMAND

A red border is created with the original imagemagick (test_collage_original) while it is white with Rmagick (test_collage).
It should be red in both cases...