how to choose the right ink for printed paper napkins?

Water-based Inks: These are commonly used for paper napkins because they are generally safer and more environmentally friendly. Water-based inks dry quickly and are less likely to bleed, making them ideal for single-ply napkins.

UV Inks: UV inks dry instantly when exposed to ultraviolet light. They offer vibrant colors and are resistant to smudging, which is beneficial for napkins that may come into contact with food or moisture. 

Solvent-based Inks: These inks are less common for napkins due to their potential toxicity and slower drying times. 

Food Safety: Since napkins come into direct contact with food, the ink must be food-safe and comply with relevant regulations (e.g., FDA regulations in the United States). Ensure the ink is non-toxic and free from harmful substances like heavy metals.
Certifications: Look for inks that have certifications such as LFGB (German Food Safety Law) or FDA approval, which guarantee their safety for use on food-related products.

Vibrancy: Choose inks that can produce the desired level of color vibrancy. UV inks often provide the most vivid colors, while water-based inks can still offer good results with the right formulation.
Opacity: For designs that require solid, opaque colors (e.g., logos or large areas of print), ensure the ink has sufficient opacity. Some inks may need multiple layers to achieve the desired effect.
Color Matching: If you need to match specific colors (e.g., Pantone colors), make sure the ink manufacturer can provide accurate color matching. 

Smudge Resistance: The ink should be resistant to smudging, especially if the napkins will be used in environments where they may come into contact with moisture or grease.
Fade Resistance: Choose inks that are resistant to fading, especially if the napkins will be exposed to light or heat. UV inks are generally more fade-resistant than water-based inks.