It's settled (by talk in comments). Your dragons are different. They are not calm, wise creatures that keep to themselves and hoard gold. They are not nuisance that steals cattle and burns occasional field. They are not a strict overlord who demands a virgin a year as a pledge of subservience. And they certainly are not noble defenders.
Your dragons are a nightmarish ravenous creatures right out of a mind of HR Giger. And you want their blood to be appropriately uncanny.
So what can we do with it?
Colour of blood (or blood equivalent) of animals we know is based on colour of oxygen carrying molecules. We (as in we the living creatures, not just humans) use a lot of oxygen and our blood carries a lot of it.
Haemoglobin is red, because oxygenated iron is red. Rust is nothing more than Iron Oxide - oxygenated iron, however iron is bound strongly to oxygen in rust, that's what rest of haemoglobin is about - to make iron attach and detach oxygen more easily. Oxygenated blood is bright red, unoxygenated blood (bound to CO2 instead of O2) is dark red.
However, Haemoglobin is not the only known oxygen carrying molecule. I present you the Hemocyanin: https://en.wikipedia.org/wiki/Hemocyanin
Hemocyanin uses copper instead of Iron. Copper gives it blue colour in oxygenated state, while unoxygenated state is colourless. For comparison Copper (II) Oxide is dark blue (at least it looks dark blue to me: https://upload.wikimedia.org/wikipedia/commons/f/fa/CopperIIoxide.jpg), however Copper (I) Oxide is red.
What does that mean? It suggests that colour of blood is primarily based on colour of oxides of metals used to carry them.
Iron is used in 2/3 oxidation states, Copper in 1/2 oxidation states, and those are most common oxidation states for those elements.
You can, with high level of plausibility, replace Iron or Copper with any metal that has 2/3 or 1/2 as most common oxidation states and blood created from those should plausibly have colour based on colour of respective oxides (2-oxide for unoxygenated Iron analog, 3-oxide for oxygenated Iron analog, 1-oxide for unoxygenated Copper analog, 2-oxide for oxygenated Copper analog).
Here are some examples I found using those lists:
and wiki articles on respective oxides.
- Cobalt, Iron analog, deoxygenated: green/red/gray, oxygenated: black
- Mercury, Copper analog, deoxygenated: brown, oxygenated: orange
- Zinc, Copper analog (it has 2 as most common state, but it has only -2/+1/+2 stats total so it might still be plausible) deoxygenated: sorry, I could not find, oxygenated: white
- Europium, Iron analog, unoxygenated: sorry, I again couldn't find, oxygenated: white but highly fluorescent (!). It used to be used in CRT screens, so how about dragon blood that glows in full technicolour under ultraviolet? Maybe Dragons can emit UV themselves and thus are afraid of certain colours of UV fluorescence? After all anything that can spill their blood is dangerous, same mechanism is why red means danger to humans.
I honestly don't know if you can use other pair of oxidation states (like 3/4 of Iridium, 2/4 of Palladium etc.), perhaps someone with better knowledge of biochemistry can answer that, or you can just ahead anyway claiming artistic license.
Whichever element you pick, you need to keep in mind that it needs to be common in your world. Dragons will have to be able to ingest enough of it, or somehow produce it themselves. Most of terrestrial live uses iron, so it's easy for terrestrial predators to acquire enough iron - it just happens naturally as they eat. Naturally, if your Dragons somehow synthesize rare element, their metabolism will slowly enrich the world with that element, so that's something you need to keep in mind too.
What you eat doesn't have huge impact on how your body works. Food is broken down to base components anyway, and a lot of amino acids, vitamins as well as fats and sugars can be synthesized if there's abundance of another compounds. For example, humans synthesize Vitamin D, and turning sugars into fats for storage and fats back into sugars for actual use is standard practice for most animals.
However, not everything can be synthesized. Terrestrial life can not synthesize elements, only compounds out of existing elements, if we need calcium we need to ingest it. Same with iron, and thus unless your Dragons can somehow synthesize other elements (perhaps with magic? Philosopher's stone anyone?) they are better off having same base blood composition and thus blood colour as other things.
But hey, who said your humans have to have red blood like we do?
Fats are flamable, be them animal, human or plant. Plant oils are easiest to use, but chemical difference between solid fat and liquid oil isn't big. Mostly, oils have some double chemical bonds while solid fats don't, that's how margarine is made: you break double bonds, attach hydrogen in thus made free spots, and presto: liquid oil turns into solid fat. You can have dragons somehow reverse the margarine process to create liquid oils, add some magic to increase flammability and you should be set with plausible flaming breath.
However, I can't help you with flammable blood. Sorry, I don't have any good ideas.
Toxicity, regeneration, and sunlight
This isn't related to colour. In terrestrial animals, regeneration is handled by blood cells which don't contribute significantly to colour.
I should note that by mammal standard, humans have fairly aggressive regeneration abilities, it may not seem like that but human body is good at staying alive despite injuries, though results may not be pretty because of permanent scars, but bleeding is stopped fast by platelet clogs, and damaged tissue is replaced aggressively by surrounding cells multiplying to fill the scar. We lack ability to regrow select limbs, but that's different from stopping bleeding and keeping what's left, alive.
In case of your dragons, most likely best pick is to have free stem cells in their blood in addition to mechanisms we posses. That can massively increase rate of regeneration - stem cells in platelet clog could differentiate into correct cell type to hasten regeneration. One huge drawback of that would be susceptibility to tumors, cancers and erroneous differentiation. If stem cells differentiate incorrectly, they could start growing wrong organs: random talons sticking out of scars, perhaps tiny disfigured appendages. Something like that could even accidentally happen in otherwise healthy tissue casing teeth inside muscles, talons in brain, scales inside arteries or even actual cancerous tumors. You could use that to limit their lifespan if you want - they could be very hard to kill, but at some point their overagressive regeneration will accumulate enough errors to for Dragon body to effectively kill itself. Kind of like human scars do negatively impact repaired tissue (lower flexibility, sometimes the can't actually do what organ needs to do and they only stop bleeding/other fluids escaping, etc.), but they sure beat being dead.
Further, stem cells could differentiate into white blood cells - those which handle immune reaction. Dragon blood isn't actually toxic to humans - it's a pathogen that actively fights and tries to kill non-dragon cells. Those chemical burns? They are not chemical. It's a trail left by dragon white cells pretending they are flesh-eating bacteria. Destroying entire limbs in matter of hours and killing a person in less than a day with only treatment being amputation before condition spreads too much is perfectly plausible, or you could dial it way back to only cause some nasty scaring.
And this leads us to photosynthesis. If dragons have chlorophyll in some of their skin cells, maybe dragon cells responsible for immune reaction misrecognise plants as dragon-cells and signal stem cells to assist in regeneration?