On a sunny day it's a dark blue, more toward the green end rather than purplish.
On a cloudy day, or stormy, it's either medium gray-green or a steel gray.
In general it tends to be a bit darker and less saturated than the other seas I've seen. (and distinctly different from the Mediterranian... I was startled at how purple that was, a "wine-dark sea" indeed!)
I just noticed you're in Jakarta... I was there in 1990, had a great time touring Java. I got a chance to see the ocean on the south side of the island, and it wasn't a whole lot different looking to me than the Atlantic on a sunny day.