Add part of Color Image Superposition (sic) section. No compile.

NOTES_ON_OPTICAL_PRINTER_TECHNIQUE.md

@@ -924,3 +924,19 @@ Gammas add or subtract in a bipack, so bipacking can adjust contrast. A bipack (
 A bipack of an original with its low contrast negative is like a darkened reduced contrast original.
 
 The bipack of an original with its high contrast negative is like a Sabattier solarization!
+
+## COLOR IMAGE SUPERPOSITION
+
+There are the same three basic types.
+
+Double exposure from positives gives additive color mixture.
+Bipacking gives so-called subtractive color mixture.
+When bipacking color negatives the extra orange mask should be neutralized by filtering.)
+
+Double exposure from negatives gives something else.
+
+For greys in the two images, combination is as for B&W.
+But for colors, not only are new colors produced but apparent brightnesses do not combine quite the same ae for B&W.
+
+The dyes in Wratten CC filters Y, M, C are similar to those in color films.
+Film colors can be simulated by packs of these filters and much can be learned about film color manipulation from familiarity with the filters and their combinations.