just used Gimp to explore contrast and saturation with the "color ..." menu. I'd suggest you pick some sample game screenshot (no JPG) that you find aesthetically pleasing and compare the saturation and values for the colours used on the floor, block and interactive tiles, too. You're likely to see wider range of values (higher contrast) for interactive things and close values with mid-saturation on the floor.
Greens, especially, tend to burn the eye when used raw on screen. Our eyes are very sensitive to shades of green much more than shades of blue or red.
Taken everyone's advice and I've had another crack it! I fully converted my palette to 9-bit and
made a ruling that only the characters and foreground-tiles can use the limited palette whilst the background-tiles must use the full palette - Changed my mind. Everything can use the full palette!
