What are the key differences in designing tattoos for color versus black-and-grey, and how do you decide which approach works best for a particular concept?

Colored pieces can look great in the short term and on white skin with all shades of color. In my personal opinion, it is best to have tattoos in black and gray, since these last forever with minimal touch-ups (it also depends on the expertise of the tattoo artist)

Light colors such as pastel colors or yellows and whites are not recommended if the skin is very dark. You may wonder why?

The ink mixes with melanin, the more melanin the skin has, the more the color of the result will change after healing and after cell division they become blurred over time. For clients with dark skin, it is highly recommended to opt for contrast over color or also to make pieces with partially dark colors such as crimson.

I invite you to see traditional Japanese art with only black and red, this is a clear example of how few colors and high contrast generate pieces for life

Color is mainly used to emphasize something within the design. While black & white designs in my opinion are my preferred option, having just a bit of red or blue color in certain areas can drastically change the overall aesthetics within the tattoo. 

When it comes to fully colored tattoo designs, its surprisingly not as challenging as you may think! So having a fully colored tattoo can be just as, if not better, than black & white tattoos. The main concern, is that different colors mean that there's many different ink pigments, meaning over time, certain colors could fade out sooner than others, causing a disproportionate tattoo composition.