Consider the type of dye
There are three main types of hair dye: semi-permanent, demi-permanent, and permanent—all of which have different lifespans. With semi-permanent dyes, the color simply sits on the hair’s surface and doesn’t tend to last more than 15 washes. Demi-permanent dyes are slightly longer-lasting (think: three to four weeks), and they typically blend with your natural color and fade out over time. The longest-lasting type is permanent, which penetrates deep into the hair and usually lasts four to six weeks.
Find the right shade
Looking for a dramatic change? Then an at-home blonde hair dye may not be right for you. When it comes to blonde shades, the safest thing you can do is stick with cool or neutral shades that aren’t *too* different from your existing color, as adding “warmth” with a box dye can result in a not-so-cute brassy hue. And if you do have a darker base tone, consider using a demi-permanent formula that’ll amp up your existing color without leaving any streaks or noticeable roots during the grow-out phase.
Read the reviews
This might seem obvious, but as hairstylist Cynthia Alvarez has told Cosmo, “Almost every person underestimates the thickness of their hair.” Make sure you check the reviews and instructions to see how much dye you’ll need based on your hair length and thickness—and when in doubt, pick up an extra box of dye just in case. “You don’t want to be left with a section of hair that’s not colored due to lack of product,” Alvarez pointed out.