To change the background color in DaVinci Resolve 18, follow these steps:

1. Open the project in which you want to change the background color.
2. Go to the Color page.
3. In the Project Settings panel on the right side of the screen, select the Color Management tab.
4. Scroll down to the Background Color section.
5. Click on the color swatch next to Background Color to open the color picker.
6. Select desired color from color picker, or enter RGB or HSL values for color you want.
7. Click OK to apply the new background color.

How to change background color in your DaVinci Resolve timeline

Alternatively, you can also change the background color by using a solid color clip in your timeline. To do this, follow these steps:

1. Go to the Edit page.
2. In the Media Pool, create a new solid color clip by right-clicking and selecting New Clip > Solid Color
3. In the Create Solid Color Clip window, select the desired color using the color picker or by entering the RGB or HSL values.
4. Click OK to create the solid color clip.
5. Drag the solid color clip to the beginning of your timeline and extend it to cover the entire length of your project. This will use the solid color as the background for your project.