Microcosm

When you're about to enter the Pod Bay at the start, and it goes to the cut
sequence, pause the game and press the following buttons:-

 Green - Blue - Yellow - Red - Yellow - Down - Green - Right - Red - Up

The Title screen will turn black and white and the cheat will be activated.
With this cheat, you're invulnerable; you can also warp around the map and
have all the guns on the next level you play. To warp around the map, simply
bring the map up.  There's a solid white block displaying your location and
another outlineed block, press either the Blue or the Yellow button; to warp
to that point, press the Green button. The only drawback to this cheat is that
all the amazing cut sequences are in black and white.  There are various other
codes which do the same thing at different parts of the game.  You can find
these by pausing the game and  pressing all the buttons until you find one
that makes a different sound to the others.  Press it then you have to find
the next one that makes the same noise.  Keep doing this until it makes that
noise and the cut sequences are in black and white.

Or try the following:-

At the start of level one, turn right and at the next crossroads push up.
Pause the game, press Yellow, Red, Green, Blue, Up and Left.  Unpause and
you'll be completely invincible.