Question
I believe my Bonsai is pine, about one year old. The color is mostly tan, not much green. I keep it indoors near north window. During warm months, water daily, feed pellets once a month. During winter, do not feed. Water about twice a week. Please tell me how to get green color back. Thank you.

Answer
To build a cold frame you will need an old window panel or a glass door.

Dig a hole on the ground of about three feet deep. Depending on the size of the plants you want to grow, maybe you'll want to dig deeper, lets say four feet.
The widht and the lenght of the hole should be the same as the window or door.
After the hole has been dug out, you could line it up with either wood or bricks.
The idea is to build some sort of a box with the top being the glass, the sides either wood or bricks, and the bottom the earth (so that it could have a good drainage).
The glass panel shoud be hinged to the structure, so that you could open and close it easily.
Place a thermometer on the wall inside.
depending on where you are, the plants should be brought in around Thanksgiving. (This is for lower New York State).
Or, as soon as the temperature starts falling down to 34 degrees (that includes the windchill factor).

To store your plants in the frame, first, you cover the bottom of it with a thick layer of leaves, or mulch, or a mixture of organic matter. That mixture will begin to decay, and that will generate a little bit of heat, which will help to protect the root system. You should bury  your plants in it, until almost touching the first branch.
NEVER BURY ANY LIVING BRANCHES OR LEAVES!
Leave some breathing space between the plants.
Monitoring the temperature inside the frame is very important.
Sun rays, after passing through the glass, leave a lots of infra- red rays that warm the inside of the frame, up to several degrees above the temperature outside.That makes the plants to begin pumping sap to the branches. At night, as the temperature falls, the branches would have become tender and would not resist the freezing.
So, it is necessary to check the temperature everyday, as never to allow it to get above 38 degrees.To control it ,just leave the panel a bit open.Don't forget to close it at night!
Whenever you see that the temerature is not going to be below freezing anymore, your plants are ready to be brought outside again.
Good Luck!