What are these red tiles in the Phoebus tileset? My entire outside area is slowly turning red i think
It's hard to say from just a screenshot, but the good news is it's probably not fire: that's generally flickering red/orange/yellow and has smoke.
Probably it's deciduous trees turning colors in the fall, but if non-tree tiles are showing it then I'd bet my last dwarfbuck it's blood.
When an entity bleeds, it creates a pool of blood - anyone walking through that pool tends to pick the blood up on their boots and whatnot and can track it around. ISTR that if there's enough of it, it can flow, too. The good news is rain will wash it away, and if it happens indoors, your dwarves will get cleaning jobs to go scrub it. If it gets totally out of control - especially inside, consider building a small, shallow (depth 2) water trench at your entrance. Water washes stuff off and forcing Dwarves to walk through it should rinse off the blood and stop them from smearing it everywhere.
Addendum to William Walker's answer:
There's a known problem/bug in the game where blood tends to multiply itself. Blood can be present on a tile in these forms, from the smallest to largest amount;
- A splatter
- A smear
- A pool
- A full tile (1/7 through 7/7 liquid).
A blood pool can create splatters when creatures track through it. Splatters can upgrade to smears by being repeatedly tracked.
Pools, smears, and splatters can stack (to unlimited amounts), they never upgrade to a full tile of blood. They can't be in the same tile as a 'regular' liquid though, which means that instead of mixing with say, water, the blood pools will move somewhere else.
Here is where the known bug that occurs when a blood pool is displaced by water in this way by, for example, rain, comes into play. It may multiply itself into multiple blood pools! (See https://www.bay12games.com/dwarves/mantisbt/view.php?id=296). Thus, the amount of blood (and other contaminants) on the map can increase without limit.
Unless you are in an evil biome or have some other source of blood with 'special properties' on your map it should not be a grave issue, as said contaminants don't affect movement. Some minor negative thoughts can be caused by the dirtiness. The only real problem can be that the amount of blood on the map can eventually overload your computer and kill the fort via 'fps death' (basically slow it to a crawl). The only known solution to this is to manually remove it via cleaning or use DFHack.