Wow, X-ray film! My recollection is that X-ray film contains chemicals which will fluoresce when irradiated with x-rays. The x-ray exposure causes fluorescence and the silver halide grains are sensitive to the visible light which is produced in this manner. I hazard a guess that the visible light is green/blue in color, so the x-ray film is probably sensitized to green/blue. Sensitizing dyes tend to be ionically attached to the silver halide grains so they should be removed in order to make room for the red sensitizing dye. You might mix the red dye sensitizing solution with ammonia which might dissolve some of the built-in dyes; it will certainly dissolve the small percentage of silver chloride grains since silver chloride is soluble in ammonia. Also ammonia might soften the gelatin which has probably been hardened--a softer gelatin will encourage the up take of the red sensitizing dye. Hydrogen peroxide will also soften hardened gelatin. All this is fairly convenient chemistry.