Open the generic browser and click on your map package.

In the grey area right click and select 'New MaterialConstant'.
Give it a name and confirm that it's going into your map package. Group is optional!

The properties window for the MIC(Material InstanceConstant) should open automatically looking like the window below.




Next switch back to the generic browser and find a base Material(or another MIC if you want) and click on it so it's highlighted.  I've used a Fluro light, from the LT package, in my example below.
Please note that not all materials have the required vector parameter for colour change!!




Now go back to your MIC and Click the Green arrow in the Parent field to set the selected material as the base for your MIC. (highlighted by arrow 1)



Next expand the 'VectorParameterValues' (bottom one, highlighted by arrow 2 above) to set the colour you want.
You can either manually set the RGBA values or take the easier, and better, option and click the coloured square and pick a colour from the colour chart. (highlighted by arrow 4 above)

Once you have your desired colour, tick the checkbox next to 'LightColor' (highlighted by arrow 3 above) to enable the change.

Next close the MIC and say yes  to save any changes.

Now the important part...Make sure you apply the MIC to an object/BSP within your map before closing the map.  If no objects reference the MIC you'll find that it will be automatically deleted if the map gets closed. Just like level previews...

Other Notes on MIC's:

*  It's possible with Kizmit to change the colour 'on the fly' on MIC's while the map is being played.  eg.. create a flickering light where the material also changes along with the lighting changes.
*  You can set the physical material for any stock material, using the 'PhysMaterial' field shown above,  so the correct footstep sounds can be set.

Hope that helps out with creating MIC's.