This is one of my main problems.
I've been working on quite a detailed sculpture drawing which would take time.
I've already coloured the pillars, but while I'm drawing the main temple above it, the pillars have already started fading in colour.
Here's a photo comparison:
Building on @Joachim's comments above, yes, the direct sunlight can affect the color and make them dull. Hence, when you are not working, you should store the paper in dark (or use some protective varnish)
Also, Do you wear any artist's glove while (A glove that covers ring & pinky finger)?
-> If you are not using it, then it might help you with smudging and fading of colors.
-> If you are using it, you might want to check the fabric and see if that's what affecting the pigments.
Also, placing some other paper to cover the already drawn part, when working on details might help.
Given:
I think it is more likely that most of the fading is due to how you work, rather than the light-fastness (or not) of your pencils.
There are lots of recommendations in this previous question for how to avoid accidentally smudging your art while working, but basically: put something under your hand so that your hand doesn't touch your art while you work.
Your colored pencils probably aren't lightfast. Look for a lightfast rating on the packaging, if there's not one, there's your problem. If there's not, keep the picture out of the sun to prevent fading. You might want to purchase high quality lightfast pencils such as Caran d'Ache, Holbein, or Prismacolor as those are the best pencils with high lightfast rating. Those are not cheap, but they won't fade in the sun either.
These should have been just comments, but they ended up too long, so here I go again, partly repeating what others had already said...
1.)
That's some very concerning fading you got there. Just in case smudging somehow contributes to it as suggested by other people, I wanted to point out that for your chosen methodology — finishing up detailed work on parts before moving on to the next — it would be possible to organize your progress in a from-top-towards-bottom, and — if you are right-handed — from-left-towards-right directions. That could mostly eliminate the situations where already finished work ends up under your working hand.
2.)
Also: never allow sunlight to directly hit your artwork. Not during creating it, and not during exhibition, when hanging on a wall somewhere. (Long-term exhibition should happen only under the protection of UV-filtering glass, or similar varnish.)
Now you could test the lightfastness (or the lack thereof) of your pencils: create two, simple, identical test pieces: this should be just some plain, colored stripes applied thickly with the color pencils you use most often, and which you want to test. To keep it economic on the materials used, it doesn't even need to be a big area.
Keep one of the test pieces in a protective dark environment, like a desk drawer or something. The other one, tape it to your window, facing outwards, where the sun can shine on it as much as it wants.
After every week of passing time, you could bring those two pieces together for a quick comparison, to see how fast the sun-exposed one fades.
If you find the rate of change on the sun-exposed one shockingly fast, you could buy a few color pencils from another brand (look for indicators/markings of lightfastness up front), and — before investing more work into actually drawing with them — just repeat the fade comparison test with them too.
(Consider that the fading test will present different results in summer- versus wintertime, due to the difference in the intensity of the sunlight and in the amount of sunny hours.)
In the end, pick the pencil brand that performs best in the fading-in-the-sun test.
Bright lights and UV rays will fade color
Dust its also a factor: since its near to impossible to stop once its settled, you'll try to remove it, and game over...
The main factor would be: sunlight. Yes. Unless they're high-quality pencils, they tend to be less likely to fade.
So...
minimize the exposure of bright sunlight and UV rays. How? by framing your art or keeping it in a dark place.
If you dont have any of this, and your house is full of windows, consider buying some cellular shades, they can protect most UV light from coming in... plus, it will give you some comfortable indoor temperature!
But what's the difference between varnish and a fixative? you may ask, well, a fixative spray is used to "fix" the drawing material (usually used on graphite, charcoal or pastel) so that it doesn't smear as easily. A varnish is used to protect it from absorbing pollutants from the environment.
