As @Eric Hu notes, a dark roux is the way to go.
It's interesting that he mentions Alton Brown, as it's his turkey gravy recipe I use.
His recipe also uses red wine, which further darkens the gravy, richens it, and adds a fantastic flavor.
I'd only change one thing: next time I'm going to make the roux and finish the gravy in a separate pan after deglazing.
The roasting pan is too big and unwieldy.
As noted in Eric's answer, a roux is just flour cooked in fat until it turns the color you want.
I usually end up a little darker than here before adding the liquids back to the pan:
I probably wouldn't go this dark unless making gumbo:
cooking.stackexchange.com/users/8522/JoeFish
Some ideas: Use a brown chicken/turkey stock.
Classic poultry stock uses raw bones, but you can make a rich, brown stock using roasted bones.
Be aware that it won't be as gelatinous as it would be with raw bones, so if you can, add some necks, backs, and if you can find them, feet.
Add some tawny port, Madeira, or dry Marsala.
In terms of technique: make a roux, add the wine, reduce to half the original volume (so the sauce isn't harshly boozy), then add stock and simmer until it reaches the desired consistency.
When roasting a whole bird, cover the bottom of the pan with a layer of onions and add a cup of sherry.
Place the bird directly on the onions (no need for a roasting rack).
Check every half hour: if the pan looks near dry, add more water.
Add the onions to your gravy and puree.
Deglaze the pan (i.e., dissolve all the yummy brown bits stuck to the bottom with water or dry white wine), then transfer it to your gravy.
cooking.stackexchange.com/users/14642/Bruce
Add a roux, ideally a dark roux, to your gravy.
This is a standard French and Cajun (which is French-rooted) technique for giving color and body to sauces.
Roux's are essentially butter or oil and flour, heated gently and stirred occasionally to cook the flour so that it darkens in color, but doesn't burn.
The darker the roux, the less thickening properties it'll have.
It sounds like you want something more like a brick-red color roux.
Some recipes make this out to be a pain, but Alton Brown has a technique that involves using an oven for gentle heating.
This is nice since it requires about 10 minutes of your attention and 1 hour and 30 minutes of baking time.
cooking.stackexchange.com/users/6818/Eric Hu
Classic technique is to use an 'oignon brule' - cut an onion in half and caramelize on a griddle or in a heavy pan.
add to the stock as it's being made.
This is similar to what @Bruce's third suggestion.
cooking.stackexchange.com/users/14590/Chris
Some people "brown" flour in a frying pan prior to making gravy.
Equal amounts (1 - 3 tbsp - more for a thicker gravy) of butter or oil and flour.
Lightly brown in pot.
I have also browned with no butter.
Our favorite way to darken gravy is to add small amounts of light Japanese soy sauce.
cooking.stackexchange.com/users/21992/Kyera
In addition to the suggestions of using a dark roux--and remember, the darker the roux, the more dark roux flavor, and the more color, but the less thickening you get from the roux--you might want to make your own dark turkey stock.
I like to roast my turkey cut up, so it is no problem to make the stock from the back, neck, giblets (except the liver), the wing tips, and so forth the day before.
However, you can always buy a few turkey wings or backs from your poultry grocer--they make wonderful stock.
Roast your turkey parts at 500 on a sheet pan (line it with aluminum foil for easy cleanup) until they are nicely dark brown.
It doesn't matter if the parts are cooked through, what you are doing is developing color and flavor.
Then, proceed to make your stock as normal.
You can also roast the vegetables for the stock if you wish--I don't bother.
This stock will give a fantastic flavor to your stuffing or dressing, as well as to your gravy.
It will also give a nice beige to light brown stock.
A shot of Worcestershire or similar also will do no harm, and may add some savory notes.
Use with moderation.
However, remember, a poultry stock is never going to be the rich brown or mahogony that a deep beef stock will be.
You should be able to achieve a rich, deep light brown color--say the color of a brown chicken egg, or a little darker, even without a dark roux.
cooking.stackexchange.com/users/14401/SAJ14SAJ
I'm just wondering why you wouldn't simply add 2 or 3 drops of Kitchen Bouquet?
If you're simply looking for that yummy colour, it's perfect.
I know, I know....
you specified not adding something like this
but I'm curious as to why.
cooking.stackexchange.com/users/35905/John Newman
Throw some flour, butter and a dark beer into it.
Even a light beer will add a fanciful flavor, IMO.
cooking.stackexchange.com/users/11015/wootscootinboogie
My favorite way of making turkey gravy is a little labor intensive.
I buy extra turkey bits (necks, backs, and wings) and roast them until they're nice and brown.
Then I simmer them in water (with a little onion and herbs, as you like) for two or three hours.
Strain and thicken with roux or corn starch (I'm gluten free).
It's GORGEOUS but definitely reserved for special occasions.
cooking.stackexchange.com/users/5958/Macromika
I recommend using potato flour* and a little soy sauce.
Use the potato flour as you might otherwise use cornflour, add the soy sauce at the end.
Wheat flour and cornflour(cornstarch) work too, but potato flour just gives the gravy a better flavour, while still being smooth.
Soy sauce might seem odd, but adding a little at the end darkens the colour and the flavour goes really well with everything else going on and just works.
Fyi: They're techniques taken from making vegetarian gravies where getting a respectable outcome is an uphill battle.
*might be called "potato starch" depending on where you are.
cooking.stackexchange.com/users/21409/Niall
When I make gravy to go with roast chicken and turkey, it is flavourful but always looks a little pale. Could you suggest a way to make it look darker without adding gravy browning?
How can I make my turkey gravy look darker and richer?
cooking.stackexchange.com
<sauce><turkey><gravy>
cooking.stackexchange.com/questions/28987
cooking.stackexchange.com/users/5449/Red Spatula
A poultry stock will not have the rich brown colour of beef stock, but you can use a brown chicken/turkey stock, brown the flour when making the roux, use an an 'oignon brule' caramelised on a griddle or heavy pan or use roasted bones to make the stock. A few drops of Worcestershire Sauce or Kitchen Bouquet will also darken the sauce without much change in flavour. You can make a brown chicken stock by roasting the bones first. You can also caramelize some onions or add some worcestershire sauce.
Use a brown chicken stock. This might be made from roasted or raw bones. You could also add a roux. Try caramelizing onions or browning some flour. Add a little worcestershire sauce or some Kitchen Bouquet.