This next step makes the colors look wrong in a technical sense, but right in an aesthetic sense. Lomo photos tend to have their color balance a little skewed and the results can vary from shot to shot, so there's no right and wrong way to do this step.
In the Layers palette, click on the Background layer. Now go to Colors > Curves and, in the Adjust Color Curves dialog, click on the Channel drop down and select one of the color channels. I selected the Green channel for my image. Now click on the mid point of the diagonal line and drag it upwards as shown in the image. If you don't like the effect, you can click the Reset Channel button and try adjusting one of the other color channels. When you're happy, click the OK button.

