UPDATED -- Thanks to Korama
Using Hivebrain's guide to hack Sonic 2 SMS, I found the UnderGround Zone pallet (16 colours) at offset 0x241FF.
29 2F 1A 3F 2A 15 03 02 01 0B 0A 06 05 0F 0B 07
I played the game with MEKA 0.71 emulator for SMS/GG and got some information about the meaning of the bits in those offsets. Colours are 8-bit coded in the way I explain: two first bits are zeros, next bits are how much blue, green and red are in the color, in groups of two bits.
Take the example of first color, $29. In binary, it's number %00101001. So we shall get this pale blue in the first color of the pallet.
In the MegaDrive, the colors are 16-bit (actually 12-bit) words, coded as %0000 BBB0 GGG0 RRR0, as only even numbers are allowed.
This research will help me in my project, now that I can take numbers %00, %01, %10 and %11, and manually replace the numbers in the offsets in Marble Zone (future UGZ in my hack) with $0, $4, $A and $E. Example:
$0B = %00100011. -- converting --> $0A0E.
This will surely be useful to me in Sonic lol pallets, just I want to do the game as similar as possible with Sonic 2 SMS.
And if Hivebrain comes here, I suggest you to update your hacking guide, because your table of Master System colors...
has now been owned. Korama has made one for you.