Will upload some more after I have made some improvements.
To answer a few questions the basic setup is as explained on an old EC. A fish tank (first remove fish ) which is filled almost to the top - I am using 8 x 10 x 8. Behind and slightly above the tank is a piece of A4 paper, in this case rainbow paper which goes from dark blue to yellow. Fire the flashgun remotely, i.e. off camera. I use the Nikon commander to do this. The flashgun is set at 1/64th output - somewhere if you search for your own flashgun model there will be a table telling you the approximation of 1/nnnnn th of a sec that this equates to.
At this time of year you need to do this is a fairly shaded room else there can be some residual movement recorded by the ambient light. These were taken at ISO 400, 1/80th @ f/22. I am now using 2 xSB900s which are angled to hit the background paper which thus reflects the colour onto the surface of the water and allows the reflection to be recorded.
Can't think of anything else I need to add to the original notes on the previous EC image from a few years back.
Post any questions, except how do you make the drops collide cos that is so "heath-robinson" as to be embarassing.