Hi. So I was just playing around with some red and blue leds today and I stumbled upon something interesting. In circuit "1", When I set it up, only the red led turned on. I tried putting six more red leds in parallel and they all lit up, but still not the blue. Then, when I tried circuit "2" they all lit up fine even when i put more leds in parallel. Why is this?
Different colors of LEDs have different forward voltage drops. A red LED Vf is about 1.8V. A blue LED Vf is about 3.0V. So in the first parallel circuit it takes 1.8V to light the red LED, but 1.8V is not enough to light the blue LED.
Connecting multiple LEDs in parallel is usually a bad idea. Even LEDs of the same production series have slightly different forward voltage to forward current ratios. Some LEDs will draw more current, and some less. This leads to differences in brightness between the LEDs that could reduce the useful life.
Thanks! I knew about Vf and all of that, but I didn't take that into account. I guess thats why they say its better to use a resistor for each led, no matter the Vf or color, they will have equal light distribution.
But even when you put your six red LEDs in parallel with only one resistor, they may not have all lit up with similar brightness. Unless they are all from the same manufacturing batch, they will likely have slightly different forward voltages anyway.
This can be used for some clever tricks. One for model railroads or similar, is to have a red and green LEDs in parallel with a single resistor except that the red LED has a switch (or other contact, such as the rails) in series. When the switch is open, the green LED lights, when it is closed the red LED lights instead. If the difference in voltages is not sufficient of itself, you can always add a 1N914 in series with the yellow or green to make it differ substantially.
Hi,
Do you have a DMM?
You can measure the difference in voltage drop.
Tom... 
This is the reason its unwise to use both red and blue LEDs in same CharliePlexed array, since two
red forward-drops are close to one blue forward drop.
Note that modern green and white LEDs are usually the same 3V Vf as blue, since they are GaN
based heterojunction LEDs typically. Older green LEDs are more likely to have lower forward
voltage, and be a lighter slightly sickly green colour too!
I have actually used what Paul said before as my indicator led so I could easily switch between the two, and now I know why you can do that. I also realized that even with the same colors, they won't have an even brightness, so I just decided to go one resistor per led and it works great. Also as Tom was talking about, it is very interesting. When I was testing them with my DMM in diode mode, there Vf's were way lower than I expected. And to Mark, I was actually reading about that when I was making my big clock. That's why you rarely see different colors for things that are getting plexed, because they aren't the same so you basically have to plex them separately. Thanks for your knowledge!
Rapid80:
Also as Tom was talking about, it is very interesting. When I was testing them with my DMM in diode mode, there Vf's were way lower than I expected.
If you use a higher current like 25mA you should see a change in Vf towards what is expected.
5V with 1K will be a current of about 3.5mA
Tom...
For pretty much any junction diode (including schottky, germanium, silicon and LEDs) the forward voltage is
a logarithmic function of current, without about 60mV change per decade of current. Usually the
forward voltage at nominal current is about 0.5V below the bandgap voltage for the semiconductor
material involved, ignoring any resistive component of the forward voltage.
MarkT:
For pretty much any junction diode (including schottky, germanium, silicon and LEDs) the forward voltage is a logarithmic function of current, without about 60mV change per decade of current. Usually the forward voltage at nominal current is about 0.5V below the bandgap voltage for the semiconductor material involved, ignoring any resistive component of the forward voltage.
Yeah.
Of course! 
MarkT:
For pretty much any junction diode (including schottky, germanium, silicon and LEDs) the forward voltage is
a logarithmic function of current, without about 60mV change per decade of current. Usually the
forward voltage at nominal current is about 0.5V below the bandgap voltage for the semiconductor
material involved, ignoring any resistive component of the forward voltage.
Yea, like he said....
Would you like to explain that further using quantum physics and relativity..lol