Probably we used not enough of the carared, but what we can do? Is it possible to use a dark fruit extract(sour, not sweet) to correct the color in the end? And what will be the effect on taste?
Fruit is not the best color agent here - the flavor will be out of character in an Irish Red.
You get the red color from a little roast barley. Take a handful of lightly crushed roasted barley (or two handfulls of whole) and let them stand in half a pint of cold water for half an hour to an hour. Strain the water, which will now be black, boil, then add it to your wort/beer. You don't need to chill - it will cool instantly on contact with the beer.
The barley will give a red color to the beer (and a very slight touch of dryness in the finish, which is welcome in an Irish Red.)
Depending upon the SRM of your roast barley, you may need to do this more than once. A handful is possibly less than you need - but you can always add more to add more color after a first try, but it's not so easy take it away!
I would go with some Sinamar. Its a product from Weyermann made entirely of Black Malt and will add color to the beer without any additional flavor.
Sinamar® natural beer coloring was patented by the Weyermann Company in Germany in 1902, and is a gluten free natural mashed coloring derived from debittered Carafa Special 11 black malt. It will help you create a dark beer with very little roast character. It is used in breweries in 67 countries today, and used to darken Dunkelweizens, Schwarzbiers, Bockbiers, Dunkel Lagers, Altbiers, Stouts, Porters, Red Ales, Brown Ales, and even Scotch Ales. If you are a beer drinker with any experience, you have consumed a beer colored with Sinamar.
Sinamar is currently popular with craft brewers in the United States making dark IPA styles, as it imparts natural malt darkness without most of the roast and bitter dark malt flavor components. Use 1 fluid ounce in 5 gallons of wort (add while cooling at the end of the boil) to add 5 SRM of color. 5 SRM is the difference between a pale blond beer and a dark golden beer. To turn a pale golden beer dark brown or almost black, add 3 to 4 ounces of Sinamar to 5 gallons.
You can drink a drop of two of this stuff straight from the bottle to verify that it does not posses much flavor at all. It would be ideal to darken up a Red Ale instantly. You should add a few drops at a time to the carboy until you get the correct color you want.
Remember, the beer is a little darker in the carboy than in the bottle, BUT you need to make sure your yeast has cleared. If not, your beer will look lighter in the carboy, as the yeast reflect light.