Chachi started out as a very light cream, and he is changing colors. His coats has almost got a gray or light brown cast to it. His nose is brown/pinkish/gray color and light blue/green eyes. What color do you think he will end up as? He is almost 5 months old now. He is a wierd combination of colors.

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They can and many do.
Hope started nearly white and now at young adult is still solid but is a bit darker cream on her back. Her nose is very black and her eyes brown.
Ruby had sabling, lost it all and then regained some of it at 6-8 months.
Eden is the same, only change is that her tan has gotten lighter. It is what I would expect because the black and tan is so distinctive from birth.

With gray coloring, non-brown eyes and a light colored nose, I'd say that there is a blue, chocolate or other dilute in his background. Do you know his dam and/or sire's coloring?

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photos-Jennifer Hague, fixyourimages.com

So they do change.
No I have no clue what his mom and dad looked like. I just answered an ad in the paper, and met her at a pet food store. She told me their names but not much about them, and I didn't know enough to even ask. She advertised him as white, but he was more of a really light cream, to me.

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Jaxx started out with only a little bit of black on his back but I noticed today he is getting more black in his coast on his back.

He looks colored very much like Ruby.

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photos-Jennifer Hague, fixyourimages.com

So I guess it is pretty typical that they change. Chach could be anything..I guess, light brown, fawn or gray looking.

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Just from your description, I might think he is a blue fawn.

Here is a link to a blue fawn chi so you can see:
imglego » Others » blue fawn chihuahua

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photos-Jennifer Hague, fixyourimages.com

Axle was like a yellow lab colour when we got him and now he is a bright tan like Ruby pictured above with a little darker sabling down his spine. He had blue grey eyes when we got him which I expected to change but luckily he still has them at 5 months, I think they're beautiful and i hope they never change. I have a Chocolate tri colour too (14 weeks old) and I expect her to stay the same much as Eden above has.