What resistor is needed to regulate the current through 2 3V LEDs in series with a 9V battery?
According to the Amazon page, the LEDs operate at 15mA, and their max current is 20mA. It is a bit confusing, but it looks like they drop 3V. Two of them would drop 6V. If you have a 9V battery, that is 3V that must be dropped across your resistor. (Of course the 9V battery will vary in voltage, but we will ignore that for now.)
To find the resistor divide the 3V by 15mA to get 200\$\Omega\$.
The color code for resistors can be found using this chart.
If you want to be a nerd, you can memorize the mnemonic Better Be Right Or Your Great Big Venture Goes West. (Black Brown Red Orange Yellow Green Blue Violet Gray White) for (0 1 2 3 4 5 6 7 8 9). For resistors with 3 bands followed by a space and then a single band, the first two bands represent the first two significant digits. The third band represents an exponent of 10 to multiply by, and the lone band is the tolerance.