You will need: a little bit of patience, a mount to practise on, a pastel pencil (I use Rexel Derwent Pastel Pencils, they have a good range of colours and they are ideally suited to this task), a putty rubber - in case you slip(!), a piece of smooth or medium sandpaper or similar to shape the tip of the pencil...and I think that should do.

First, lay the sandpaper flat on your work surface, then shape the point of the pencil by rubbing it gently on the sandpaper. What you are trying to achieve is two flat sides that taper to a point so that when you come to the inside corners of the mount, the pencil will reach into them. Now, work with the mount in one hand, face uppermost, and the other hand holding the pencil, underneath and coming up into the aperture. Start anywhere that is comfortable and work in any direction you like, but the idea is to use the pencil to colour the edge by gently but firmly rubbing it along the bevel, keep it flat against the bevel face. Repeat until you have an even application of colour. You will need to experiment a little - keep the pencil at an upward angle at all times, this way it should never come into contact with the face paper of the mount.

Be adventurous...mix the colours; smudge them; apply a different colour to each side, or pairs of sides; experiment, have fun!

The above technique can also be done with a brush and acrylic paint but it is a bit tricky to angle the brush so that you don't get paint on the mount face. A pointed smallish (No:2 or 3) brush with stiff bristles seems to work best. And don't mix the paint too thin. Practise on a bit of scrap mount first.