CHIKYU KAIHO GUN ZAS Look at that last screenshot! This Game Boy game has transparencies and uses more than 4 shades! How is it possible?! Cool graphical tricks aside, this is a really good vertical STG with old-school horizontal sensibilities, not many games like that, especially nowadays!

If I remember for ZAS it's doing the parallax and likely other things with temporal blending- alternating backgrounds between different frames (making positive use of LCD blurring).