Step 3: Making the color mixing chamber
The light from the LED passes through the first diffuse surface, gets reflected and scrambled inside the tube and passes through another diffusor, scrambling it even more. This reduces the brightness of the LEDs a lot, but makes for a nice and smooth spot light.
Ok, so cut the 20mm pipe into pieces of the same length of about 6-10mm. Use sand paper to make them nice and flat.
Use the dremel tool with some polishing paste to polish the inside of the pieces of tube.
Glue the diffuser (milk glass, scotch tape...) to both ends.
Glue the whole mixing chamber on to the LEDs. I used epoxy glue for this, but it does not stick too well to the plastic of the LED and I often had to do it twice or three times because it broke off during the assembly of the lamp.