As
@thedoc298 said, each LED is different. It used to be that you could predict the forward voltage by the color of light the LED emitted. This is certainly NOT true for "white light" LEDs, which use a wavelength-converting phosphor to emit light of a different color than the LED illuminating the phosphor. And it gets more complicated from there. So, your best bet is to get a variable current-limiting power supply, capable of at least five volts open-circuit output, and use it to forward bias your LEDs. If you have a lot of them, bin them according to forward voltage at a specific test current (20 mA is usually safe) and label each bin with the color the LED emits. Common "colors" are invisible infra-red; visible red, yellow, green and blue; invisible ultra-violet; and visible white. You cannot go by the color of the case to identify an LED! Measure the forward operating voltage.
Back in the day, when LEDs were just becoming available for sale, I bought a selection in panel-mount packages with various colors of plastic lenses: red, yellow, green, and blue IIRC. All of the LEDs worked, but they all emitted red light! I don't think the vendor realized this was a limitation. They apparently just thought they could replace incandescent lamps with LEDs and keep the same package. I didn't know any better either. Back then, if you wanted colored panel lights, they had to be incandescent lamps with color filters, NOT red LEDs with color filters.
To this day I am still partial to incandescent panel lamps, despite their limited longevity and sensitive to vibration. I think they just "look" better than LED lamps. But <sigh> paying customers disagree, so it's been a long time since any of my designs "featured" incandescent lamps. Oh, well, I could still use them for hobby applications if I could afford the light bulbs, which I generally cannot. LEDs with different color outputs are cheap and readily available. I am becoming quite attracted to the itty bitty surface-mount types that don't even look like LEDs until they "light up" with application of power. Way cool that look on a densely populated digital logic board. A real PITA to hand-solder them to a board.