atropupureum is the standard japanese red maple. Are the ones you are looking to purchase grafted? A lot of atros are just cutting, and so have no grafting scars, they can be found in most of the major nurseries. I have 3 of these, 2 in the ground, and one in a grow tub shown in this thread:
http://forum.bonsaitalk.com/showthr...anese+red+maple
this link also has a nice example:
http://www.yamadori-bonsai.de/c/06/01/02/07/82.htm
Notice that this japanese red maple does not keep it's red color.
The two I have in the ground in a shaded area look like the one above, with green leaves, and some red coloring.
The one in the tub, still gets full sun, so it is still red. Next month it will have to go to the partially shaded area, so will lose most of it's red color as well.
However, come autumn, all 3 will turn back into brillant red for a few weeks before shedding their leaves.