To be effective, a dye must initially have 2 properties. It must first be colored opposite to the wavelength you wish to sensitize the emulsion to, therefore a green sensitizer will be magenta in color and etc. Second, it must adsorb onto the surface of a silver halide grain. These are the two basic properties.

The third property, acting after the first two, is the ability to pump the energy absorbed by the dye into the grain as if the photon were hitting the grain, so a blue or UV sensitive grain would become green sensitive by adding a magenta colored dye to the grain. The emulsion is still blue sensitive, so you end up with blue-green (ortho) sensitivity by adding a green sensitizing dye.

The two simplest dyes are chlorophyll, a weak red sensitizer, and erythrosine a fair green sensitizer. Using erythrosine will give you an ortho emulsion from a blue sensitive emulsion if added properly. Erythrosine is the active ingredient in some food dyes and also in the old antiseptic Mercurochrome.