Illustrator - Colorants of raster item


I work in a printer house. I need to save thousands of Illustrator files in pdf. To do this no problem.
But before saving each file, I must delete or change the colour of some embedded pictures.
In Illustrator (CS6) embedded images appear as “Raster Item class” or “Group Item class”. They are bitmap or gray images colorized with Illustrator. If I look at their properties, most of the time the Colorants property is {“Gray”} !! even if I applied a spot one from my palette.
How to know the exact spot color I used to colorize images ?

I have had similar issues with this in AI before now. I’ve only ever been able to colorize using CMYK and NOT able to apply a spot by script. If I recall correct I found NO solution.

OK, thanks.
If I get the solution I’ll let you know.