Open the scene
open the surface editor
Look for the color controls
Click on the T button to the right of color
here you will see the color gradient being used.
You can click on the arrow head on the left of a color
Then use the color picker to select the color.
You can click on the x on the right of the color to remove it.
remove all but 2, one at the top and one on the bottom.
make top yellow and bottom blue.