I use a simple circuit to notify user the reverse biased voltage to prevent some issues. Vcc can be -18~+18V, and two LEDs have exactly same reverse voltage (5V), but they have different forward voltage (because their LED wavelengths are different, blue one has 3V, red one has 1.8V). Will this setting cause a problem? In my test this circuit not failed but I'm not convinced. I think if the LED voltage is within the range of the reverse voltage, it's just okay. Is my thought right?
Will this setting cause a problem?
No it will not.
I think if the LED voltage is within the range of the reverse voltage, it's just okay. Is my thought right?
Yes, you are right. As long as you are operating the LEDs within their safe limits, they will not be affected
There are some readily-available, multi-colour bi-directional LEDs on the market. Please follow this link: Led-link
Because the two LEDs are back to back the maximum reverse voltage applied to one will be the forward voltage, Vf, of the other.
... blue one has 3 V, red one has 1.8 V ...
When the blue LED is lit there will be 3 V in reverse across the red and when the red is lit there will be 1.8 V in reverse across the blue. This is how the bi-directional LED is intended to be used and it will be fine.
As far as I remember, bi-directional LEDs are available with the same colour in both directions. These are suitable for AC indicators and eliminate the requirement for an external diode.