Well, personally I really love Gianas GfX. However, maybe you (Rex and MacDeath) can explain whats wrong with the gfx, or how they can be improved.
The best thing would be just to provide an example (better cloud ... f.e.). 
Would be great if you can use the same set of colors, but it's not a prerequisite.
From memory I would say that mixing Green and dark-red is not that good...
I'll try to make some mockups, but if you could supply us with Tiles sets and sprites sheets... or more pictures with no compression (PNG...) and more accurate resolution on PC, it would be fine IMO...
Tiles sets could be sweet so I may know more exactly the limitations or imperatives/specs... to make proper suggestions then.
I'll look at what is available... seems you didn't put any "decent" picture on the net...
The pictures there :
http://cpcwiki.eu/index.php/A_Tribute_to_the_SistersGreat, it looks like a proper CPC screen, but it can't be reworked on a PC...
Best would be to give us a few pictures in real actual CPCresolution...
Of course you can double pixels (as wide pixels of Mode0 are wide...lol) as it is easy to re-convert to simple pixels or simply rework as it is...
But please, don't put a 768x540... more like a 384x270 with no scanlines effect...
Also, using a 270 vertical display... I believed that depending the heating of the monitor, some lines could be in overscan zone (out of the actual screen...)...
BDC-IRON told me that the more the monitor heats/warm-up, the more the displayed vertical resolution is... (of course this vary slightly...but...)
wouldn't it be easier or safer to try a slightly inferior vertical resolution ?
Vertical 256pix may be better as you would be more sure that the monitor display all the playfield everytime... And this may save some Video Ram too...
Anyway, this is an ambitious project : a full screen mode Mario clone...
And a lot of work seems to have been done already.
Good luck.
Also I hope you didn't find my comments too rude, it wasn't intended...
I know a bit how difficult it is to work graphics for a game... but having some suggestions from others can also be halpfull...
Ok, perhaps i found more usable pictures on CPCrulez...
I'll try some suggestions anbd mockups...
Ok a first re-try, but done quite quick...not worked a lot but a few stuff are shown perhaps...
I used CPCcolours set on my Paint.net application, so the exact colours may be different from other stuff... but this remains approximately faithfull...
Ideas are :
As I told you : mixing dark green and dark red (pseudo-brown...as CPC has no real Brown...) is not a good idea, because, well, it's mixing red and Green... which is bad.
Of course the dark red is darker than dark green so may be considered a proper shade, but it is not really...
Just go from White to black, put 2-3 greens between, and dither a bit more...
--earth : use orange and dark Red in order to get a brown effect (perhaps not exactly as I did...) and add a few pixels of yellow to highligh a bit more, and also perhaps a few pixels of red to in order to get a complex and colourfull feeling...
But pleaaaase : never mix the orange and the Pink...!
So many CPC games did this and this simply don't mix well IMO... (but you didn't, actually... it is just a reminder...
)
--Green tube : more contrast and only use of greens +B&W...Yellow too, perhaps (I did not but some studies may give interesting results)... also the shape and shades, relief and dithering are to be slightly improoved perhaps... (but a metallic effect is quite hard to get...)
Your version for exemple goes from a light blue to greens to dark red to black...
Black and white are great in that they mix with everything given a proper dithering... and it also gives more contrast...
--Some blues are to be removed/changed , maybe other inks too... per exemple what i call the Blue-grey... replace it by a 3rd green... This pseudo grey is not brightly coloured enough for a cartoonish game like this.
If the level is to include some sort of vegetations, it is far better to get another Green... also concerning the green Tubes (mario like...)
--Also the purple from the clouds... please use a blue too... and mostly : swap the light blue and the white... only this is a lot IMO...
--replace the Pink by something else... perhaps... the Amstrad pink don't mix properly with almost all the rest of the palette IMO...
And the "mountains" is perhaps too blurry on the "outside"... need to get it more as in Super Mario 3 or super Mario World (4, on super NES) perhaps...
On thing to keep in mind is that Giana sister is mostly a C64 game... but C64 palette is somewhat... special.
Don't be afraid to go full CPC.
Many dithering can be redone because CPC palette is somewhat superior on some matters...
Appart from true Greys and real brown, CPC is superior in every other stuff of the spectrum... especially if you seek brighly and vivid coloured stuff...
As a matter of fact, we simply almost have twice colours actually (but the brown and Grey...)
And because C64 has 3 greys and 2 browns (orange is light brown, and dark brown) on 16 colours... many colours actually have 3-4 time more variations on CPC.
I didn't put all this, I did this very quickly but maybe you'll get the points while comparing the pics...
Of course those are minor suggestions, the final choices are up to the actual graphist of the project.
And the few we could see seems to already be a good basis.
concerning the way giana jumps...
The ideal way would be to get the same feeling as in Rick Dangerous...(bouncy as hell...)
check up the rick 128+ version : Fano put a special option enabling to jump on the heads of opponents (without dying again...lol...)...
perhaps asking him how the jumps were managed on rick may be helpfull...
