To change the color of a layout grid in Figma, which is set to red with 10% opacity by default, follow these steps:

1. Click the Layout guide settings icon next to the layout grid in the Design panel.
2. Click on the color swatch that appears in the settings window.
3. This opens the color picker, where you can choose a new color for your grid.
4. You can also adjust how see-through your grid is by changing the opacity.

This lets you customize the look of your grid to improve visibility against different backgrounds, enhancing your design workflow and accuracy.