I guess in a similar vein, talking about 4 colours makes me think of mode 1 on the CPC, which I'm really not a fan of for graphics. But the situation in AiT2 isnt really just 4 colours, it's the 3 fixed + 1 variable as you point out, so it shouldnt look anywhere near as plain as the games I think of when 4 colours is mentioned!

Static screen could look better on A8 because 'quad wide' underlays PMs gives you that 'colours/luminances' you want/you need...

C16 game was build with their colour clash problem... now build the same on A8 without colour clash but on an A8 way...
Get your soft sprite routine running and this time and for C16 portings you really just need 128 chars=1Charset.

Why? Because on A8 you can have 1Charset for the Top&Bottom Lines and Status Area that leaves you with just another Charset just for the Playing Area...
ADVENTURES IN TIME can be on A8 with just one Charset on the Playing area (including soft sprites+shifting)...
And that in just 5colours Mode Antic4:


And this leaving you 'free cycles' because you will not need to start your PMG underlays on the BadLines:


And yes, I said you could have it, but it's up to you (up to you not personal but for alll, in general ...):

(And yes, I just don't choose this Walls 2nd Level because it's more, or seems lesss difficult because I can also get the others ... maybe on the next days, or if someone is interested)