I Have Three Colors: A "What Am I" Riddle

Question

Here is the riddle:

I have three colors,
each with a different meaning.
When I'm old they become dull,
but when I'm new they are gleaming.

I help keep people safe,
but sometimes they hate me.
You won't see me in a forest,
but you will in a big city.

Some of me have short lengths,
and some of me have long.
And in the time that the longest of me take,
you could sing a very long song.

When my colors change,
people get happy or sad,
and when you guess what I am,
you will be very glad!

Good luck!

Answer 1

You are

a traffic light.

I have three colors,

Red, yellow, and green

each with a different meaning.

Red is a no-go, yellow is to prepare, and green is go.

When I'm old they become dull,

Excessive sunlight make the color-coating of the traffic lights fade, or alternatively the light bulbs can lose their strength.

but when I'm new they are gleaming.

Shiny new lights are nicely visible.

I help keep people safe,

Traffic lights keep you in order.

but sometimes they hate me.

You don't like to wait when it is red.

You won't see me in a forest,

Hardly.

but you will in a big city.

Indeed, at major traffic junctions.

Some of me have short lengths, and some of me have long. And in the time that the longest of me take, you could sing a very long song.

The time of changing color depends from place-to-place, and also from color-to-color. Red takes forever!

When my colors change, people get happy or sad,

Time to start your car!

and when you guess what I am, you will be very glad!

Indeed. Although I would not be surprised, if this would be a duplicate. We should go and check.

Answer 2

You are a

Stop light.

The three colors are

green, yellow and red. Green means you can go, yellow means to proceed with caution, and red means stop.

You keep people safe

by controlling the flow of traffic.

But some people hate

to be stuck at a red light, especially if it's a long one.
There are no stop lights in a forest, only in a city.
There is no standard length of time for a light to change. Some of them can take a very long time.
People are happy when they have the green light to go, sad when they have to wait at a red light.