Electronic – SMD precision resistor color code (0805)

To start, Some of your math is a little off.
For the red LED, if you are using a 5v supply and the red LED needs 20ma and has a voltage drop of 2.1v, then you need a limiting resistorstrong text of (5-2.1)/.02 = 145 ohms.
For the green LED, you need (5-3.2)/.02 = 90 ohms.
For the blue LED, you need (5-3.1)/.02 = 95 ohms.

Assuming that these resistors cause equal LED illumination, and that the light intensity varies directly as the current applied to the LED, then you need to reduce the currents to the green and blue LEDs as follows:

For the green LED, the current needed is (20ma x 20% =) 4ma. For the same voltage drop, the new current limiting resistor required is (5-3.2)/.004 = 450 ohms.
For the blue LED, the current needed is (20ma x 15%) =) 3ma. For the same voltage drop, the new current limiting resistor required is (5-3.1)/.003 = 633 ohms.

Obviously, if the assumptions are not accurate, the calculations will also not be accurate. If more accuracy is required, then you will need to use the LED's data sheet.

1. Generally if an accuracy is given as 1.5%, it is assumed that this means ±1.5% unless otherwise stated. Thus you should not substitute one for the other.
2. Precision can be better than accuracy. In your example, however, since the readings are near the edge of the circle and spread around, the precision is not much better than the accuracy. If the readings were closer together then the precision would be better than the accuracy.
3. Precision could be expressed using ± meaning that the readings will cluster around the average reading within that percentage.
4. Again, if you express precision without the ± sign, then the ± will generally be assumed. It is better to use the ± sign and avoid any ambiguity.