For the color of the frog monster’s lips, we can choose a color that adds a playful yet slightly monstrous touch to the character. Here’s a suggested color addition to the existing palette:

 <Frog Monster Lips>
1. Lip Red - 
                            
                            #C74B4D
                         - A vibrant and slightly glossy red that adds a bold and playful look to the frog monster's lips.

This Lip Red ( #C74B4D ) will stand out against the green tones of the body, enhancing the character’s whimsical and quirky appearance.

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

 <Frog Monster Theme>
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. Mud Brown - 
                            
                            #4B3D27
                         - A deep, earthy brown that reflects the muddy grounds of a bog.
4. Lily Pad - 
                            
                            #A8D8B9
                         - A soft, light green that captures the essence of lily pads floating on water.
5. Monster Eye Yellow - 
                            
                            #E1C300
                         - A bright yellow that evokes the eerie shine of a frog monster's eyes.
6. Pond Water - 
                            
                            #2A6B7E
                         - A cool, dark blue that represents the mysterious depths of a pond.
7. Bug Bite Red - 
                            
                            #D94F3D
                         - A bold red that symbolizes a frog monster's appetite for insects.
8. Night Sky Purple - 
                            
                            #4A2C6D
                         - A deep purple that adds a hint of mystery and night-time ambiance to the palette.
9. Tongue Pink - 
                            
                            #D85E8B
                         - A bright, playful pink that adds a fun and slightly slimy look to the frog monster's tongue.
10. Eye Green - 
                            
                            #A2D600
                         - A vivid and luminous green that adds a captivating pop to the frog monster’s eyes.
11. 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 explore any other features of the frog monster, or discuss how to combine these colors in your design?