I have not worked on colistin (Polymyxin E), but I know the situation with Polymyxin B. This antibiotic is not substrate of efflux pumps, I think the same will be with Colistin
I hope it is useful: Contribution of EmrAB efflux pumps to colistin resistance in Acinetobacter baumannii. Lin, MF., Lin, YY. & Lan, CY. J Microbiol. (2017) 55: 130. doi:10.1007/s12275-017-6408-5
Now a days, efflux pump has become a common media to vomit any clinically important antimicrobials (CIA). I have worked on FQ resistance efflux pump. Recently several researchers are working on colistin efflux pump, and they are getting interesting results with the identification of novel colistin efflux pumps.
What bacteria do you work with? colistin resistant mechanism generally mediated by lipid a modification by phospoetanolamine. It will lead reduction of colistin affinity to LPS. But some report also mentioned about some efflux pump inhibitor like CCCP able to reduce the MIC of bacteria and in vitro as well
CCCP is not efflux inhibitor, it is a compound deenergizing cells. Efflux pumps are dependent on proton motive force, and, of course, do not work in the presence of CCCP. So, reduction of MIC can be also a result of general deenergization (inhibition of transport, ATP synthesis).