I've had this problem, too. The early growth, after a very bad winter this year came out green, then we had a severe late frost but the later growth has come red. My acer is now two-tone - green smaller leaves on top and normal sized red/purple on lower growth. The only factor I can pinpoint is the terrible early season weather and I will be interested to see any other answers to this question.

Red Japanese acers do look much more green at this time of the year. I'm looking out of the sitting room window at two of them and they are not the vibrant red they were earlier in the year. All part of the changing seasons, the leaves will start to turn (go brown) in the next month.

One of my purple acers grows like a weeping willow on a standard trunk. The foliage is really thick and I find that the lower leaves - which get less light are green. I doesn't seem to have any effect on the plant as it is healthy enough.

The warmer it gets, the greener the leaves get--and they don't always change back. Also, the leaves get greener in deep shade.