For high-end PE lines, why don't Japanese companies produce dark-green color? I saw all rainbow colors except dark green.

Seaguar and Sufix have you covered there, generally in larger diameter braided line.  

Where Japanese X-braid excels is in smaller threadline sizes, e.g., PE#1 is 20-25-lb breaking strength.  

Pretty easy to argue in line this small, you're more interested in being able to see it rather than a need to camoflage it.  

On the recent Favorite Finesse Braid thread, @redmeansdistortion pointed out the 10-m color changes in threadline braid are very useful for counting off the amount of line you're adding to your spool.  The color change is there for this function, not for decoration (offshore jigging guys measure their drop depth counting color changes).