How do you change Keynote template color?

You need to change the slide Masters if you want all slides to have the same colour. In the View menu, choose the menu item "Show Master Slides", then edit the Masters. Any slide that uses a particular master should take on that colour.

Note that the background of the Gradient theme is actually an image. If you want to be able to manipulate gradients directly, you can set the background of the Master directly. This approach will let you create a uniform gradient between two colours. If you want much more control over the gradient, including using arbitrarily many colours and controlling the blending, you can instead cover the Master with a rectangular object, then in the Graphics Inspector choose for the Fill "Advanced Gradient Fill". To add additional colours, simply click in the gradient area. You can get very fancy gradients with this approach.