Meditation object: Use color seeing when eyes are closed Sometime when I started to meditate (eyes closed) I can see some waves of colors (yellow, blue, gray). Is this a good meditation object to concentrate one those colors?. 

I feel like it is quite easy to concentrate on the color than another object (Breath, feelings, etc). Is this a bad practice?
The wildmind website does list seeing colours as one of the signs of progress. So you should obviously find that encouraging. However it does caution


  But it’s best not to pay much attention to those lights or they will
  turn into a distraction and slow your progress.


This accords with my own experience. While various strange things during meditation is fascinating, too much fascination will mean that you are not focusing on whatever your object of meditation actually is (breath, visualisation etc...). Absolutely dig around in them and have a little investigate but ultimately return to the practice you are doing. Just my experience of course.