For the most part plasters are for interiors only. You do have some other alternatives. If you have unlimited money to spend, look into tadlect. This is not a DIY material. I am a professional faux finisher and worked with a tadlect applicator and I decided not to do it any more.

You have several other practical alternatives. You could simply have the wall parged or stuccoed. You can have pigment added to the material and it will maintain it's color.

You can paint it a dark color and cover it with trellises and grow vines on it. Made sure to have some evergreen vines if it is visible from your windows in the winter.

If it's a prominent feature in your garden, consider doing your own mosaic on it. You could also include a wall fountain made from the mosaic material. If you are on a tight budget, you can make the mosaic out of marble and tile scraps that people get rid of on Craig's list.

If the top is really ugly, you can get slabs of stone from a stone yard cut for you, and just mortar them on yourself. Have them cut the pieces into lengths that you can pick up yourself.

Parging or stuccoing is probably the way I would go...I think this is what was done on my neighbor's retaining wall. I was hoping for a DYI alternative, but I don't like the idea of paint...I'm concerned it will fail and make things worse. There is a concrete patio right up to the base, so vines would have to be in pots, but the light situation back there is difficult.