From here on it is about shifting colors. If you want Red sky, then you can skip this step. If you want Blue sky you will need to swap the Red and Blue colors in your image. It is easiest to do this with the Channel Mixer by creating a layer and setting Red to 100% Blue and setting Blue to 100% Red. Leave Green 100% Green. Unfortunately the Channel Mixer is not a standard in Elements 3.0 and needs to be downloaded and installed as an action. (More on this below). Elements 4.0 does not support adding this action, so ironically Elements 3.0 is better in this respect. Yes, you could probably get a similar effect using Hue/Sat function to shift Hues, but Channel Mixer is MUCH easier.
(If you have the full version of Photoshop, you already have the Channel Mixer.)
Photoshop Actions that will run in Elements 3.0 (not in 4) that will be very useful are the Channel Mixer and Curves. Visit the following web link and read carefully about how to install the applicable actions.