Hey there, thanks for your interest! In the context of applications, in this case monochromatic art would be acceptable as samples (whether a single hue or greyscale) as long as you're showing us your use of incorporation of color and shade in some way that will allow us to evaluate your general skill level. When it comes to creating for the zine, should you be accepted, you would of course have to incorporate color into your illustration, which is why we are encouraging you to pick pieces that can hopefully both display your style while also showing us that you are still able to work with color even if it isn't traditionally what might be considered "fully colored."