THE REVERSAL COLOUR PROCESS
In this case, a tripack similar to normal Negative/Positive Colour Films is used.
1. 1st Developer - a black and white developer produces a negative silver image
2. Reversal Exposure - light fogging of the undeveloped silver bromide
3. Colour Developer - a colour developer that develops the positive silver image and positive dye images from the silver bromide not developed in the First Developer.
4. Bleach - converts all the silver [both negative and positive images] to silver bromide.
5. Fix - removes all silver bromide into solution leaving only the positive image.
Some colour reversal processes have many more than four solutions. (Kodachrome, for example)
Stop bathes, clearing bathes, activators [to speed up the next chemical reaction], stabilisers [to protect the dyes from premature fading] and hardeners [to harden the emulsion for high temperature processing] have all been used, and many still are. As many as twelve different stages may be needed to process some colour reversal materials.
Almost all the colour films used today fall into these categories except for the Kodachrome films and some years ago, Dynacolor.
Kodachrome is "substantive" process and a reversal material. The couplers that produce the dyes during colour development are soluble and are present in the developer solutions. There are three colour developers, each with one coupler and several reversal exposures are used to separately reverse a layer at a time followed by the relevant developer. The dyes produced are insoluble. This process can only work as a reversal process.