All this, so fast, than the eye doesn't catch the blinking
Internally, the WS2812 LEDs does this for you. When you program the color to red=20, g=0, b=0, the WS2811 controller chip inside the LED will rapidly pulse the red on for 20% of the time. WorldSemi (who makes the chip) doesn't specify the rate their chip pulses the LED. I've seen claims between 400 to 2500 Hz.
Because the WS2811 chip inside the LED is already pulsing the LED, if you try to also turn them on and off rapidly, you're likely to get strange interactions between the 2 rapid update rates. But if you're only wanting to do this to regulate the amount of power you'll use, there's absolutely no need. The chips already provide variable intensity by modulating pulses. All you have to do is set the color you want.
Power budget-wise, you have have 500 mA to work with, if plugged directly into a PC or powered hub. Let's assume you're not going to be plugged into an unpowered hub, which limits you to only 100 mA.
Teensy needs about 30 mA. If you're using WS2811 chips, each consumes about 0.9 mA just for the chip, regardless of whether the LED is on or off. So with 60 LEDs, that's 84 mA used up, leaving you 416 for the LEDs. Also consider these numbers are "typical". They can vary somewhat, especially as the temperature changes, so if you're going to mass produce something like this, it's important to consider either the worst case (which is usually quite depressing how much it limits you), or at least a worst likely case, perhaps for indoor only use.
Each color in a WS2812 LED uses approx 16 to 18 mA. Obviously if you turn them all on 100% at the same time, you'll go far over the 416 mA limit. But you could turn on all 60 LEDs in such a way that each uses about 6.9 mA. That could be turning on only the red part at 40%, which would be r=102, g=0, b=0. Or if you want them all yellow, perhaps set them all to r=51, g=51, b=0.
While working on this project, I would highly recommend using a powered USB hub. Hubs are a lot cheaper and easier to replace than computers. In theory computers and hubs are supposed to have over-current protection, but better to risk the cheap hub.
It would also be a really good idea to add a tiny current sensing resistor in series with the power going to all those LEDs. You can get 1% accurate 0.1 ohm resistors at places like Digikey, usually for under a few dollars. Then just connect a voltmeter across the LED. As you develop your project, keep an eye on the voltage. If you get close to 50 mV, or go over, you'll know your configuration is using too much current. You might be tempted to use your multimeter's current mode, but it works with a series resistor inside the meter, and usually the value is higher than 0.1 ohm, which means you'll lose more than 50 mV. Buying a 0.1 ohm resistor is much better.
