okay i made a little progress but there was something i miss understood. it seem while there are 2 lines of 15 colours available in the palette any one object can only use colours from a single line..
as a result I have had to move a little bit away from the machine while I rework every other item I had drawn, it also seems will have to rework sonic tails and knuckles sprites a little at some point due to the colours being moved around on the palette
i have ended up focusing on reworking Robotnik him self so he and his flying machine use only line 1 colours. this frees up line 2 draw the vehicle that his flying machine sits into.

I now have 3 possible versions of him, had trouble getting his moustache which was originally brown to fit into line 1's colours.
and also his flying machine(with brown moustached Robotnik that didn't fit in line 1's colours):

i am going to have to spend some time working out a few of the mega drives quirks before i draw the vehicle..
ill post up a newer version of that when I've fixed that..