Because accessing the Build mode is a necessity for building, you will have to do so to start designing your house. It is possible to easily access it within the game by interacting with the mailbox automatically present at the front of your plot in Bloxburg. You may also press the 'House' button, which is placed on the right side of the screen when standing on your plot. Subsequently, to paint/color your wall, simply press 'F' on your keyboard or click on the 'Paint' button to open the palette.