Alright, for those who eventually are interesting in this, i figured out a few things on my own and through this very good

Video [explaining Voltage drops in series/parallel circuit].

So these japanese PDFs for the LEDs in the Starter Kit tell us a few things (

Blue LED,

Red LED,

Green LED,

Yellow LED):

As i understood the part which says "forward voltage" is the minumum voltage the LED needs to turn on and that is seperated into a Minimum, Typical and Maximum voltage. The values are:

blue: [min:3.0V, typical: 3.2V, max: 3.4V]

red: [min:1.9V, typical: 2.2V, max: 2.4V]

green: [min:2.0V, typical: 2.2V, max: 2.4V]

yellow: [min:unknown, typical: 2.1V, max: 2.5V]

There is a schema of my circuit attached.

~~So the voltage we supply to the LED has to be higher or equal to the min voltage of that LED (blue need 3.0V). As we dont know the overall current yet i was assuming a Voltage drop of~~

`5V-3V = 2V`

at the 220ohm resistor which then tells us a current of

`2V/220ohm= 0.009A`

[/s]

I assume that the voltage drop at the first 220 ohm resistor mast be at a maximum of 2.0V for the blue LED to work as it needs a minimum of 2.0V. Well, as you can see in the schema there is a second series of LEDs which can be any color but has to have again the minimum Voltage supply.

But the first series obviously has some sort of resistence which causes the voltage to drop again.

And thats why, if we assume that the first series gets exactly 2.0V and there is a slight drop of 0.1V inside the first series the second series has a supply of 1.9V.

If we look at the table at the top with the minimum supply values we can see that its not possible to light the blue LED anymore. The green one needs exactly 1.9V at minimum and i could observe exactly that, but the light it emits is really really weak. And the same goes for the yellow LED which minimum Voltage value was unknown but seems to be around ~1.9V too.

What i dont know is, how this is all related to the current at the LEDs because in series in current is the same everywhere, but in parallel it gets split.

Maybe that is the reason why 2 blue LEDs in parallel can work but if i put one red and one blue LED in the first series, only the red LED emits light and the blue doesnt light at all.

If you could give me a bit feedback on this i would appeciate it