Something you might notice in our mockups and color libraries is that we never use 100% white or 100% black. And we recommend you do that as well! Why I hear you ask? Well simply because nothing is ever really pure white or pure black. There is ALWAYS going to be some underlying hue, light or shadow that gives it a hint of color, no matter how subtle.
So when we put together our mockups the black we use is #252222. This color is made up of greys and very subtle dark reds. So when you apply this color to your shirt it still looks black but it's more realistic. You can of course ignore this and use #000 :)
Same goes with the white! We use #F9F7FC. This is made from very light greys and subtle purples giving you a more true to life representation of what a white t-shirt looks like. This also helps it stand out from your background if you're using a light or white background.
Once again feel free to disregard this information and use #FFF But at the end of what day that's why you're here isn't it? To make the most real life mockups you can. So why go so far and then stop right before the finish?