Welcome to the FX codes
Blue codes are old, they can be found everywhere or in a few places.
Red codes I've found in mags and sites but I can't get them to work. (they are probably fake)
Green codes- I don't have this game so I can't test it.
Orange codes are discovered by me, you might find them elsewhere but they were here first.
Sonic 1
Enter these at the title screen.
Level Select + Sound Test - Up, Down, Left, Right,(ring) hold A, Start
Extra Options During Game - C, C, Up, Down, Left, Right(ring) or Up, C, Down, C, Left, C, Right(ring)
When paused    A=Reset    Hold B=Slo-Mo    C=Frame Skip
Debug + Invincibility - After Extra Options During Game code Hold A, Start
B=Enter/Exit Debug    A=Change Item    C=Place Item
Stupid Sonic - Hold A+B+C During Demo
Extra Maze Area (get 6 same color emeralds) - In the Special Stage, enter debug and hold left a few seconds, exit debug and you will drop into a place with a 1up icon, other icons and lots of chaos emeralds, collect an emerald to end the stage, look for your favorite color and get it. (easiest to do with blue)