By default, Flash Cart uses the colors from your Theme settings to match your branding. You can can choose to use Custom settings to select the color of each component in the theme editor.
STEPS
In your theme editor (Customize):
Click to open App embeds in the side panel.
Click the Flash Cart tab to open the settings.
Choose whether to use colors and styles from your Current theme or Custom settings.
The Current theme will use a default color scheme if theme colors cannot be automatically applied.
Click the color swatches to select the color you want for each component of the cart.
To add a specific hex code color, click on the field to enter code (e.g. #FF0000)