this will tell you how to make a rainbow color changing solar light.

Step 1: Supplies

soldering iron, color changing led, solder, old solar light with batteries.

Step 2: Dissasimbuly

remove the screws holding the 2 parts of the lights together.

Step 3: Asimbuly

solder your color changing led in combine a cople if you want. hwever this will lesten it's runtime. for the led i used it has to be the 3 volt type with 2 batteries in it.
i got my led at http://www.goldmine-elec-products.com/products.asp?dept=1245
item number G13897 is what i used ues any one on that page you like.