Depending on the size of the flash (the one I was using was a Vivitar 283) you could find the sample swatches from Roscolux or Lee Filters and then just tape whatever colour you want to the front of the flash. The photo shows I used to go to gave away sample packs in long 3.5 inch by 1.5 inch strips (Lee Filters Numeric Edition), or 3.5 inch by 3 inch patches (Lee Filters Cinematographer's Edition). There were some samples of reflector material in some but most were colour gel samples and they fit perfectly across the front of my flash with every colour and shade you can think of. Best of all they were free then, but you may have to pay for them now.
I have used two different coloured gels on the front of my Metz 60CT4 for something different as well.