I only have one theory as to why it's not working and I don't know how valid it is. It sounds like the clear yellow is an enamel, which would explain why it dosen't mix with future, which is an acrylic.
Now, I don't know for sure if Tamiya makes an enamel clear yellow, but they seem to think they do http://www.tamiya.com/english/products/list/enamel_1.htm
Here's a list of tamiya's acrylics http://www.tamiya.com/english/products/list/acrylic_1.htm . For some reason, the acrylics and enamels have the same part number. so somewhere, something is screwy. Either tamiya has the same part numbers for 2 different paints, or tamiya's website is wrong.
If I find anything concrete either way, I'll chime in again (unless someone comes up with a more reasonable solution to the problem).
-Fred