We have a fellow in our lab who worked on QDs (quantum dots).
So I have learned that most QDs can be excitated under wave length about between 400-600nm.
It means that you can use one certain wave length to excitate several different QDs which are distinctive in emission wave length (thus different in color). But the drawback is that you can not use two or more QDs in order to see colocalization such as nucleus and membrane molecules of the same cell. 

 Baowei Peng
1954 Huashan Road
Pharmacy School
Shanghai Jiaotong University
Shanghai, 200030
China