Pav are some colours harder to do? Or to be true, as it were?
Please, ask away! I certainly don't mind your avatar popping up. Three part answer incoming!
1) First and foremost, what you probably care the most about-- colors as they pertain to the customer. I'll start off by saying that you only want to choose colors that are lighter than your skin tone. We get black people coming in, wanting red and yellow and all kinds of craziness. It's not a racist thing, but a factual thing, if you want a color to show up well on your skin, then your skin can't be darker than that color. Now, that's not to say the color won't be present at all, but it certainly won't be "true." I absolutely love tattooing pale people. The colors show up so vibrantly.
Also, there's not one color that hurts more than the others. All colors will feel the same going in. At least they should, if you have a decent artist, which brings me to part two...
2) Colors as they pertain to the artist! I'll keep this short, as it shouldn't ever matter to a customer, but as artist, we really have to be careful with how we're putting in certain colors. Yellows and reds in particular can be difficult to get into the skin. If you're not careful and spend too much time trying to get them in, you can really destroy ("hamburger") that tissue and cause serious scabbing/scarring.
3) Colors as they pertain to laser removal. Now, if you've done your homework, you shouldn't have to worry about this. However, if you've made a mistake somewhere along the way and want to be rid of it, you might have to deal with this. Different colors respond to different wavelengths of lasers, so depending on how colorful the tattoo you want removed is, you may have to undergo more or less sessions, depending. Black is the easiest color to get rid of, whereas blues/greens are the most difficult. Most artists say when it comes to tattoos, the darker the color is, the easier it is to remove, but that's not entirely accurate, and is a bit of a generalization. If you're that interested, google will help you get a far more in-depth knowledge of laser removal than I'm prepared to go into, here.