For the outline color of the frog monster in the Gravity Falls art style, we can choose a color that will provide a clear contrast to the vibrant Swamp Green ( #319179 ). Here’s the suggested outline color addition:

 <Frog Monster Outline Color>
1. Outline Dark Green - 
                            
                            #1A3D3D
                         - A dark, muted green that provides a subtle yet effective outline, enhancing the shape and definition of the frog monster.

This Outline Dark Green ( #1A3D3D ) will create a cohesive look while still allowing the main Swamp Green color to stand out.

Here’s the updated color palette including the outline color:

 <Frog Monster Theme with Outline>
1. Swamp Green - 
                            
                            #319179
                         - A vibrant green reminiscent of the swampy habitats where frogs thrive.
2. Swamp Green Shade - 
                            
                            #1E5B4D
                         - A darker shade of swamp green for shading.
3. Swamp Green Shadow - 
                            
                            #1A4D4D
                         - A deep shadow color that adds dimension to the frog monster's body.
4. Outline Dark Green - 
                            
                            #1A3D3D
                         - A dark, muted green that enhances the definition of the frog monster's features.
5. Mud Brown - 
                            
                            #4B3D27
                         - A deep, earthy brown that reflects the muddy grounds of a bog.
6. Lily Pad - 
                            
                            #A8D8B9
                         - A soft, light green that captures the essence of lily pads floating on water.
7. Monster Eye Yellow - 
                            
                            #E1C300
                         - A bright yellow that evokes the eerie shine of a frog monster's eyes.
8. Pond Water - 
                            
                            #2A6B7E
                         - A cool, dark blue that represents the mysterious depths of a pond.
9. Bug Bite Red - 
                            
                            #D94F3D
                         - A bold red that symbolizes a frog monster's appetite for insects.
10. Night Sky Purple - 
                            
                            #4A2C6D
                         - A deep purple that adds a hint of mystery and night-time ambiance to the palette.
11. Tongue Pink - 
                            
                            #D85E8B
                         - A bright, playful pink that adds a fun and slightly slimy look to the frog monster's tongue.
12. Eye Green - 
                            
                            #A2D600
                         - A vivid and luminous green that adds a captivating pop to the frog monster’s eyes.
13. Lip Red - 
                            
                            #C74B4D
                         - A vibrant and slightly glossy red that adds a bold and playful look to the frog monster's lips.

Would you like to discuss how to apply this outline in your design, or do you have any other elements or features you’d like to explore?