ofxAssimpModelLoader alpha

How can I change the alpha of a 3d model? If I call

        ofSetColor(255,255,255,alpha);

Before I draw I can change the alpha but then all the textures are white, (whereas when I call this before drawing an ofImage the colours of the image are correct but reducing the alpha value reduces the transparency).

How can I fade in and out a textured 3D model while keeping the colours?