Now try horizontal and make a game, not a demo

Each game has its own needs As I see the screenshoot, I think that light and clear background could do enough fine with monochrome (and dark sprites). Then you have something smooth, as this kind of games usually needs.  But yes, depending on how enemies move, you could do this and that... Same as depending on how many of them are on screen, you can add a second sprite for each.  The advantage of 2nd method is that, for the programmer, it is easier to deal with, so he can focus more on the game and less on the tricks I think, it's just my opinion.

Way to go, Timmy. Imho, your fist goal should be to get used to the platform, that's new to you: once that's done, then there will be time for perfection

I vote for small sprite flickering ( the msx has built-in flickering sprites ;-) ). If you need to know how to do it, the trick is simple:
suppose you had to show 6 sprites on a scan line:

A B C D E F
sprites ( E, F) will be dropped, not shown. Each frame you could fill the VRAM table (the SAT) where sprites attributes are kept by rotating (shifthing) the sprite planes. So the next frame will be

B C D E  F A
now, F and A are dropped, next frame you fill with

C D E F A B
and so on, until you reach again to the ABCDEF configuration.

this is called sprite plane rotation. There are other tricks, of course but i think this is the more simpler to code.

you can also "preserve" from flickering some planes (usually the main charater) so your configuration become:

ABCDEF
ABDEFC
ABEFCD
ABFCDE
ABCDEF -> again!

this will ensure that AB planes always are shown and others will flicker

 

Nice to see Spectrum programmer making an MSX conversion, Timmy!  It´s interesting to read what Spectrum programmer has to deal with when "porting" his game to MSX.

Here is one more helpful hint for making your player and enemies more colourful. Try Colecovision game "Galaxian" on bluemsx emulator, you will soon see how the programmers had done enemy ships so colourful, it is a nice "trick" indeed and I think you can get new ideas by watching it for a while.

Please show and tell us more as your converting goes on!