When wavelength of light is stretched, so light becomes red which is seen as shifted towards the red part of the spectrum.

This is similar to Doppler effect of sound. When the source and observer are moving when they come closer frequency heard by the observer is higher and when they move far away frequency of the sound heard is lower.

There are many galaxies in our universe and when light reaches to earth from these galaxies the wavelength of the light is shifted to longer wavelength and becomes red or we can say red shifted.

The red shift observed in a distant galaxy is not due to Doppler effect but rather due to expansion of the universe. It means everything is moving far away from each other.

Scientist claim that there is a mysterious dark force present in our universe that is pulling Galaxies apart.

Hubble Telescope Observation

In 1929, Edwin Hubble discovered that a more distant a galaxy is from us the faster it recedes in to space. That proved that universe is expanding from all the directions.

The rate of expansion of the universe is called Hubble constant.

Conclusion

So if today you are observing a distant star which is of yellow color and just after 50 years if the color fades and become little reddish it proves that universe is expanding but 50 years is a very little time to observe this effect.

So everything that is looking yellow ultimately shifts to red color and fades away after few billions years because everything is moving further away from each other so in a few billion years light wave will be stretched enough to enter the non visible part of the spectrum.