An LED is a diode that has a small fixed voltage drop. If you connect it directly to a power supply that has a voltage higher than the voltage rating of the LED then the LED will draw an extremely high current and will burn out. Most ordinary LEDs have a max continuous current rating of 30mA. They are recommended to operate at 20mA.
A red LED is about 2.0V so with a 12V supply, 10V will be across the 1.5k current-limiting resistor and Ohm's Law calculates the current to be 10V/1.5k= 6.7mA which is not very bright. For 20mA then use R= 10V/20mA= 500 ohms which isn't a standard value so use 510 ohms.
A blue or white LED is about 3.5V so the current-limiting resistor will have 12V-3.5V= 8.5V across it. The value of a current-limiting resistor for 20mA is R= 8.5V/20mA= 425 ohms which isn't a standard value so use 430 ohms.
A red LED is about 2.0V so with a 12V supply, 10V will be across the 1.5k current-limiting resistor and Ohm's Law calculates the current to be 10V/1.5k= 6.7mA which is not very bright. For 20mA then use R= 10V/20mA= 500 ohms which isn't a standard value so use 510 ohms.
A blue or white LED is about 3.5V so the current-limiting resistor will have 12V-3.5V= 8.5V across it. The value of a current-limiting resistor for 20mA is R= 8.5V/20mA= 425 ohms which isn't a standard value so use 430 ohms.