Saki was the same. When I got him at Petco, he was a light greyish color, but I could see a "barely there" reflection of lavender in his tail. I figured maybe he will grow up to be more lavender? When I got him home and he got all warmed up, his body gleamed more silverish and his tail went several shades darker. Within a week his body was a copper/silver mix, and his tail was deep purple with copper reflection. In age his body has become a greyish purple, but his biggest color change was within the first couple days after I brought him home.
Same goes for my newest little guy, Soren. When I got him this week, he was bright green but only if you had a good light to see him with. But once he was warmed up in his tank, his green became more obvious without great lighting :)
If you think about it, we're not too different. We blush and our faces gets more red when we are embarassed, angry, upset, or even excessively hot or cold. When we don't feel well or we have had some kind of shocking experience we become pale. Whenever I look at bettas in the store I concentrate on those "barely there" tints of color in their fins, because when you take them home, chances are those colors will come out more :)