Using a layered shader, have two layers. The first is red and the second in blue. Make sure "Compositing Flag", which is located directly underneath your layer color and transparency sliders in the attributes editor, is set to "Layer Texture" instead of "Layer Shader." Make sure the transparency for both layers (the red and blue) are as high as you can get them on the slider. (have the slider all the way to the LEFT) Using the same right-click and set key technique I explained about in your other thread, animate the transparency of the red layer to fade out. This will reveal the blue layer.