Dark tattoos can throw off Apple Watch's heart rate sensor

Here's one factor to take into account if you're still undecided about getting the
Apple Watch : it doesn't seem to play well with tattoos. Several users revealed online that their devices act wonky when worn on a tattooed arm. The heart rate sensor wouldn't read their pulse, and the watch wouldn't detect direct contact with the skin, causing apps to stop working and repeatedly ask for the passcode. To confirm if inked skin really does affect the smartwatch's functions, iMore has decided to perform a series of tests. "[W]e're inclined to agree with those early reports," the publication writes, "if your tattoo happens to be a solid, darker color."
iMore has discovered that dark colors like black and red affect readings the most to the point that the watch can't even register being in contact with skin. Lighter colors cause some heart rate misreadings, as well, but they don't render the watch useless. It makes sense when you think of how the sensor works. See, in order to read your heart rate, the Apple Watch flashes green LED lights hundreds of times per second. Since red blood absorbs green light, the device can detect the amount of blood flowing through your pulse when the LEDs flash. The presence of ink that blocks light can then hinder the sensor's ability to detect skin and the amount of blood flowing underneath.
You can switch off "Wrist Detection" to prevent the device from asking for a passcode every time the screen comes on. However, that solution isn't perfect: for one, it switches off Apple Pay, as well. Of course, you can always wear the watch on your other arm if it's not tattooed, but you can also just wait for the next iteration if an update can't fix the issue.