Is it possible to keep transparent shades light in cloisonné work or, in general, to make transparent colours lighter?

Yes, it is. To make transparent colours a shade lighter, just add a soft flux before firing your work. This has the same effect as adding water to your water colours. I like using thompson # 2007 for this.

A problem linked with this question is the darkening of transparent shades in cloisonné work as the thickness of the enamel layer increases. To prevent this from happening, just top up the enamel in the cells with a clear flux for silver. I use Thompson Flux for silver # 2020 as this is absolutely uncoloured.