It may have been clearer if we had been using Build Colour rather than Build Vector in these experiments but the whole think was kicked off with passing a vector (Voronoi 3D B Vector), to the cloud layer' density input and the vector has stayed.
To understand what is going on you need to think of the three inputs X, Y and Z of the Build Vector as equivalents of the Red, Green and Blue inputs of a Build Colour. The Density Shader input of a cloud layer is expecting a single value scalar. When a cloud layer is given a three component vector or built colour the density input treats the three components separately and creates three cloud layers one Red (X Input), one Green (Y Input) and one Blue (Z Input). If you use the same function for all three inputs of the Build Vector the three cloud layers will be the same and you will get white clouds in the normal way.
With the nodes setup the way you have you are creating two cloud layers; one Green, controlled by the output of the Multiply Colour and one Magenta (Red and Blue), controlled by the output of the Subtract Colour. Without being able to see the settings in your Density Fractals I am having to guess a little here but I am assuming that the Multiply Colour produces a more widespread cloud layer with a low density and that the Subtract Colour is producing a more confined high density layer. Where these layers intersect you are getting high density cloud with a slightly pinkish colour (Magenta plus a little Green). Elsewhere you are seeing either dense Magenta cloud from the Subtract Colour or low density Green cloud from the Multiply Colour.
If you replace the Build Vector with a Merge Shader; use a Constant Scalar (0.5), as a Mix Controller to begin with, you should get the effect you are after. You will probably need to fiddle with value of the Constant Scalar in order to get the right mix.
