Since you have a top loader, Cheer Powder is a good inexpensive option - unlike Cheer liquid, the powder version has no optical brighteners added, and those are usually responsible for color fading. It's a little hard to find offline, but in my area all Targets stock large 120 load boxes for around $15.

Here is a link that might be useful: Cheer Powder at Target.com

Ecover and Oxi Clean powder also don't contain optical brighteners, there are a few things you can do to slow fading(besides using cold water):
1. In the first wash use white vinegar and let soak in cold water for an hour, this will help set the dye.
2. Turn the garment inside out.
3. Use less detergent.
4. Use the delicate cycle or a shorter wash cycle.

What recorders said but with Warm water.

You might also try 2 rinses. I feel like leftover detergent fades colors just like it makes whites dingy.