Why the green is different

Where the volts go

Each bar is the pin's voltage, split between an LED and its resistor. The coloured part is the LED's share, and it barely moves as you slide the pin voltage. The orange part is the resistor's share, which is everything left over, and only that sets the current.

With 1 kΩ, a volt across the resistor is a milliamp through it. So the length of the orange part, in volts, is the current in milliamps.

The green keeps more for itself

Every LED takes a roughly fixed voltage before it conducts, and the colour decides it. The three datasheets give these ranges:

At the few milliamps a 1 kΩ resistor allows, the red and yellow take about 1.9 V and the green about 2.7 V. From a pin, that works out as:

These are worked out, not measured, and a real LED can sit anywhere in its range. The shape is what holds: the green always gets the least, and on a 3.3 V board it gets less than half of what the others do.

It still looks as bright

Less current does not mean less light here, because the parts are not equally efficient. The yellow's datasheet gives its brightness at 20 mA: 70 to 175 millicandela. The green's gives its brightness at 5 mA, a quarter of that current, and the range is 175 to 430 millicandela. The green's dimmest part is as bright as the yellow's brightest, on a quarter of the current. The eye also happens to be most sensitive to light near yellow-green.

So on an Uno the three look much alike. On a 3.3 V board the green is visibly the dimmest of the three, but plainly lit. Why it is dimmer on 3.3 V works the same arithmetic for the XL LED's blue.

Safe on every board

The largest current here is about 3.1 mA per pin, and about 8.5 mA for all three together from an Uno. An Uno's pin may supply 40 mA, an ESP32's about 20 mA at its default drive, and a Pico's 4 mA at its default. Every board in this book is well inside its limit with all three lights on.