Describe the Colour of Raw Liver
Solution 1:
In the context you've given, I'd personally use the word livid (Dictionary.com). Livid is often used to describe the color of fresh bruises and it also has the meaning of being speechless with fury.
Although the picture you've posted doesn't appear that way, in my experience, raw liver is pretty similar in color to the deep reddish purple of fresh bruises. I think livid not only conveys this, but also adds a sort of distasteful, gross feel that would match your context.
If you think that 12-year-olds won't understand that context, you could consider using bruised instead.
Solution 2:
I would describe it as a bloody color, but my English teacher would probably say it's Burgundy, which is the answer you're probably looking for.
Update: I would like to point out that liver is in fact a color, as @BiscuitBoy pointed out.
Solution 3:
Naturally, as an internal organ it looks rather bloody, so I'm primarily going to suggest sanguine
Of the color of blood; red; ruddy: as, a sanguine complexion; the sanguine francolin, Ithaginis cruentatus; specifically, in heraldry, same as murrey.
The Century Dictionary and Cyclopedia 1890-1914
It is a bit problematic though since blood color can come in many shades depending on the state it's in: It can be very bright when it's thin and fresh or very dark when it's clotting and despite being very apt by definition for many contexts, I have never seen anything directly described with this word.
The alleged synonym Murrey is quite helpful though since the C.D.C. specifies it's the color of a mulberry, which should be relatively easy to observe in reality:
Of a mulberry (dark-red) color.
n. In heraldry, a tincture of a dark-reddish brown, also called sanguine, indicated in heraldic representations in black and white by lines crossing each other
The Century Dictionary and Cyclopedia, 1890-1914
Given that these are colors of heraldic tradition, you might also be able to examine a coat-of-arms with the color. The Heraldry Society's Education Pack P.D.F. pack may be helpful. Although the C.D.C. considers these words synonymous, which makes sense given that berries may have been used for such pigmentation, Sanguine seems to be a brighter color than Murrey according to the H.S. P.D.F., so despite the semantic appeal of Sanguine, Murrey would be more accurate if both words don't refer to the whole spectrum of reds blood and mulberries can be.
For your convenience and as a contingency for if the Hearldry Society's website goes offline, here's an image file with both colors, with a Sanguine backdrop and a Murrey heart. It looks fairly close to me. It also seems like the colors of this website's unbolded and bolded links:
Speaking of contingencies and convenience both of the above Wordnik links also contain entries in The American Heritage Dictionary, 4th edition which is a newer and presumably less rare book than the multi-volume C.D.C. if you need to verify with a print source.
Solution 4:
Ironically, "liver" is used to describe a color of dogs' coats. In Spanish and French, marron, which means chestnut, is also used to describe shade of reddish-brown. It's rough equivalent in English is maroon, which can be anything from a deep red to a reddish-brown. Yet maroon conjures up the color of a car more than it does the color of a liver. "The maroon liver" seems cartoonish.
In a sentence written for a young audience, you may want to consider going beyond color, for example, "Mary pushed the dense, reddish-brown lump of raw liver around her plate." (Mary was, no doubt, wondering why the liver was not cooked prior to serving.)
As a 12-year-old child, I would have immediately understood "reddish-brown" and been encouraged to look up "dense", thus learning a new word.