Some pigments are going to fade faster than others. It's a simple fact! If you have any concerns about the colours in your paintings fading you can rid your paint box of colours that have crummy lightfast ratings for a start. 

There are scientific labs that exist internationally that provide these ratings. The common place that provides ratings for north american art supplies is the ASTM (The Amercian Society for Testing Materials). These scientists fire light at the pigment to see how long it will take to fade. Imagine a poster hanging in a shop window that has faded into pale greens and blues. The light breaks the particles of colour into smaller and smaller pieces until it's faded completely to white. Those old posters show us that certain colours fade slower than others.

The lightfastness rating from the ASTM is listed as 1 - 4 but in Roman Numerals so: I, II, III, IV.

Look at the tube of paint below made by the company Golden. You can see the label "Lightfastness II". This is how it will appear on your paint tubes. Unless of course your paintings have been tested by a different scientific company that runs similar tests.

Here's what the rating system is:

I     Excellent lightfastness

II    Very good lightfastness

III   Fair - non-permanent 

IV  Not lightfast a.k.a. this is considered a fugitive colour as in - it will disappear

*If you are concerned about fading colours then stick to paint tubes that have an I or II rating.

Image source [1]
Image source [2]