NeoPixel led WS2812B doesn't shine when given equal RGB levels
I suspect the problem is an electrical one: The WS2812B dim by using PWM, i.e. switching LEDs on and off. Switching causes the current draw to vary and causes noise on the power line. Now, if two of them have the same setting and switch at the same time, noise adds up causing an error in the diodes' controller. Try adding a 100 nF (or similar) capacitor close to the LED between VCC and GND.

Regarding timing, the WS2812B are much easier to control than the timing diagram suggests. My experience is, as long as you make sure that the high period is between 0.25 and 0.5 µs for a '0' and longer than 0.7 µs for a '1'; and the low period is shorter than the reset time, everything works well. 

The following is unproven, but I strongly believe that the logic inside the controller to receive data is a simple "wait for input going high. wait 0.6 µs. check if input is still high ('1') or low again ('0'). repeat."