Anionic and cationic dyes must not be mixed because their opposite charges would cause the dyes to coagulate and precipitate from solution. We see colour when light shining on a material is partially absorbed by the molecules of that material. Only a portion of the spectrum is reflected from the surface and so instead of seeing white light, we see a colour. The amount of black in a colour is described in terms of the colour's brightness. Hence, the brighter of two reds will contain less black than the duller hue.
Strength refers to the concentration of dye in the material.
A strong colour will have more dye colecules per unit area than a pale colour. Hue and brightness are intrinsic properties of the dye. They can be varied by mixing dyes. The dyer determines the strength by the quantity of dye added to the dyebath. Colour interact in subtle ways.