The first successful fixer is sodium thiosulfate. In the 60’s when ammonium thiosulfate became available it became the fixer of choice because it operated about twice as quick. Nowadays ammonium thiosulfate is the norm, however, either will work. Now color film and color paper use carefully selected organic dyes. These dyes blossom if the pH (acidity – alkalinity) is correct. If not, they can revert to a colorless state called leuco (Greek for hidden). The fixer solutions we use contain the fixer plus an acidifier and a preservative. The job of the fix is to purge the film or paper of undeveloped salts of silver. In the color process silver salts must be removed along with flakes of metallic silver. If the silver were to remain, the metallic silver would veil the colored dye.

Since the fixer cannot readily remove metallic silver, in the color process, the fixer is preceded by a bleach bath. The bleach attacks metallic silver and coverts it to a silver salt. The fixer bath following the bleach bath is now able to rid the material of the veiling silver.

The color paper process combines the bleach and the fix making a single solution called a Blix. The C-41 process continues to use two bath bleach and fix. Many home color film processes also use a Blix (single solution bleach + fix).

The bottom line is, if your process uses the two step method, then if it is an emergency, you can try substituting a black and white fix.

Would I do it? Likely not.