I'm not 100% sure of the Mega Drive 2 details at the moment, so I'll leave that to one side for now.
The following information applies to the Mega Drive 1, which contains a 24-pin Sony chip labelled CXA-1145. (You'll need to remove the big heat sink that's attached to the two 7805 voltage regulators in order to get at it.) This chip is the composite colour encoder which converts RGB to PAL or NTSC video.
The first thing you need to do is desolder three pins of this chip - pins 5, 6 and 7 - and lift them clear of the board. (Alternatively you can just cut the pins close to the board and lift up what's left.) The idea is to disconnect these three pins from their respective board holes.
Pin 7 is easy to deal with; just connect a wire between it and pin 1 (Ground). This is the PAL/NTSC select pin; connecting it to ground enables PAL output.
Now you'll need to go to the electronics shop and buy 4 components:
4.433619MHz Crystal (a.k.a. PAL Colour crystal)
15K Resistor (metal film type)
30pF Capacitor (ceramic disc)
50pF max. Variable Capacitor (a.k.a. 50pF max. Trimmer)
If you can't find a capacitor of exactly 30pF, use a 33pF one. The trimmer max. value isn't so important, so long as it's less than about 100pF (and more than 40pF) it should be fine.
The crystal value is critical, however.
All of these components are non-polarised, i.e. it doesn't matter which way round you connect them.
Now, the next step is to build the circuit. The rather crap diagram here:
shows how the components should be connected together; how you go about this is up to you.
Personally I would get a small piece of prototype board or Veroboard and build the circuit on that. You can just solder the legs of the components together (as shown in the diagram) if you prefer, but it won't be as neat. (Ensure the component legs aren't touching anything! Wrap in insulating tape if necessary.) Make sure you can access the variable capacitor once you've put the heatsink back on, since you'll need to adjust it...
OK, refit the heatsink, and switch on your MD with a cart inserted. You'll probably get a black and white image. Now simply adjust the variable capacitor with a screwdriver, in small increments, until you get a nice colour picture. Make sure that it works in both 50Hz and 60Hz modes.
Remember that I haven't tried this myself; you do it at your own risk! I'm simply going by the CXA-1145 spec. sheet.