Blood - not just another pretty color

Most writers have a basic grasp for how to handle their character's bodies. Arms break and mend, bruises run a gamut of colors, and cuts tend to bleed.
For simple injuries, those descriptions will cover it. Problems arise, however, when a tiny cut bleeds gallons, or a two foot hole through the midrift fails to kill.
We're here to cover those things, find out if they're a problem, and if so, how to fix it.

First the disclaimer: I'm no doctor, so be sure to always check your facts relating to the specific injury you intend to inflict on your character.

Let's start with flesh wounds. The most common misconception seems to be, if a character hits a hard object, that their hand will immediately bruise. "Mark punched the wall, then plopped into the chair to glare at his bruised knuckles." Not a great example, but will do for the purpose.
Bruises do not form instantly. They might come quickly, within minutes, or more likely hours, but never instantly.
What will happen is the injured flesh reddens and swells. Depending on the severity, it may only rise that far. Mark might see redness and swelling, but a bruise won't appear for another hour or so.

Bruises are usually purplish black in color, fading to shades of gray, green and ochre yellow as they heal. Bruises over areas of skin where the veins are close to the surface may appear bluish green. That's because a vessel has either been torn and there is minor bleeding under the skin or the vessel walls are weakened and bulging. The blue-green color is unoxygenated blood. Those type bruises will appear quicker, swell more and be particulary tender to the touch.

So what about quick bruises? They have their place as well.
Hemophiliacs, those people who bleed profusely at the smallest injury such as tiny nicks, can acquire quick bruising.
Bullets will cause bruising within a few minutes to hours. A lot of people mistake the greyish area around the entry point to be bruising. It's usually powder burn.
Blows to the head, or to blood rich areas unprotected by bone or hard muscle can bruise quickly. Note that quickly is still defined in minutes.

A bit on bullet wounds in general. Bullet wounds can, but don't always bleed immediately unless they've pierced a vital organ or blood supply. The nature of guns and bullets usually causes them to sear or cauterize the flesh they impact, especially at close range. They're hot coming out of the barrel, and cool and slow with distance. Distance is more likely to cause the bullet to become lodged in the body, but a lodged bullet can also slow bleeding time if it's blocking the blood supply.

Blood - not just another pretty color.
"The floor was red with blood" or "his pants were red with blood". Not exactly wrong statements, but misleading.
Fresh blood is red. Really, really red. Kind of a brilliant scarlet with undertones of blue when looked at through a tube or as a beaded droplet on the arm. That fresh red color is blood full of oxygen. And well, like everything, blood decays.
Blood begins to 'die' as soon as it's spilled. A small droplet of blood decays sooner than a large area such as what's on the floor.
The oxygen slowly leaks out of the blood cells, called coagulation, and leaves behind a residue of iron. The color of iron is orange and brown. Rust. Old dead blood that's been sitting around without oxygen to infuse it, is brown. It can be thick dark brown mahogany, or shades of orange smeared across a paper. But it's no longer 'red with blood'.
Washing off dried blood in water, such as the shower, does not turn it red again. It's still brown.
Blood is also sticky. Yes, that means the fresh stuff, too, given a few moments to coagulate.
Most of us at some time or other have written along the lines of, "their feet slipped on the blood covered floors". Note that this isn't reasonable unless they're splashing in puddles of the stuff, or it's the beginning of a battle and the blood spilled is fresh. If it's the aftermath of the battle, their feet are probably peeling up from the floor with wet sticky sounds.
Guts, and other internal body matter is more likely what they are slipping on. Most internal organs, not all, but most of the ones spilled in battle, tend to be a pinkish grey color, and squishy, thus easy to slip and fall over.

Other notes on blood:
Bleeding from the ears can be caused by depressurization such as in an airplane or underwater, a Very Bad Thing, by the way. Nosebleeds and similar injuries can cause very minor blood from the ears and/or eyes because of the way the ducts and paths are set up. It does take a lot of pressure to cause such a reaction, though. Blood in the urine or other elimination can be signs of many things, but for our purpose of injuries, is usually a sign of kidney, liver, or intestinal damage.

So - what did we learn?
Bruises aren't instant.
Blood comes in many forms.
Lack of these instant forms may seem less exciting than using words like red, or bruised, but they'll make you're story more believable. Think of them as challenging expansions on the experience that you want your story to give your readers. Unless they're squeamish, they'll thank you for it.