The mobile version of your Builder website comes with a white and grey color scheme. If you'd like to personalize that color scheme, you have complete control over the colors you'd like to use.
From the Builder control panel, hover over design and click on mobile.
Click on the colors tab and you'll see your current color settings.
To change any of these colors, either replace the HEX codes, or click on the color swatch and select the color you'd like to use.
Make sure to click on save changes when you've updated your colors to your preferences.