Which variety of Acer is it?

Not all Japanese Acers have red leaves we have several in our garden with soft yellowy green ones. As Bamboo says which variety did you buy?

If it is supposed to be a red-leaved variety, adjusting the sun exposure may help. If it is in fairly deep shade, giving it some filtered sun, or morning sun, might deepen the color. If it is already in strong sun, giving it more shade could help. I would also double check the fertilization--a feed with a full suite of micronutrients, and a moderate level of potash could improve the color. Note that some "red-leaved" varieties only have red leaves for a few weeks after sprouting, then they change to green with bronze tints or accents.