The cap tells you three things, and the colour tells you only one
Most guides say "look for the blue cap". That is the weakest of the three clues, and on plenty of cars it is wrong — black caps are common, and some engines hide the bottle under a plastic cover with nothing blue visible at all. The three clues that are actually reliable are all on the cap itself.
It pulls off. The washer cap is a push-fit lid you lift or pull, usually with no thread at all. Coolant and brake fluid caps screw on, and some of them need a quarter turn past a stop. It has no level marks. A cap with MIN and MAX moulded into it, or a dipstick attached, belongs to a reservoir whose level actually matters — coolant, brake fluid, power steering. Washer fluid has a level, but it is not critical, so the cap carries no scale. It has a fan of spray over a windscreen. That icon — a rectangle with water lines fanning up at it — is close to universal, and it is the only cap in the bay whose icon refers to glass.
The failure mode nobody expects is not confusion, it is haste. You are at a service station, the bottle is open, the cap you are holding is roughly the same size, and the coolant reservoir is right there. Screen wash in the cooling system dilutes the antifreeze, which lowers its boiling point and raises its freezing point — so the engine can overheat in summer and the block can crack in winter, and neither shows up for months. Screen wash in the engine oil is worse and faster. Screen wash in the brake reservoir is the most expensive of all: brake fluid tolerates no water and no contamination, so the whole system has to be flushed and refilled. All three are the same mistake with three different bills.
Tap water is free and costs more than the fluid
Tap water works for a week and then starts to build up. It carries calcium and magnesium, and those precipitate out in the nozzle and the hose as scale — the jet goes from a fan to a squirt to a dribble, and eventually to nothing. It also has no detergent and no lubricant, so the blades drag over a drier screen, which wears the rubber edge and puts fine scratches in the glass. Washing-up liquid is worse in a different way: it is alkaline enough to attack the rubber, and it foams, so a screen that looked clean becomes a smear of bubbles in exactly the light you need to see through.
Proper fluid is a few pounds for two litres and does three jobs: it carries a detergent that shifts insect and tree residue, a lubricant that lets the blade glide, and — the part that matters in winter — a freezing point. That last number is the one to read on the bottle, and the rule is at least 10 °C below the coldest your area ever gets, not the average. Somewhere that touches −15 °C needs −25 °C fluid. If you have been running summer fluid and the cold arrives, the bottle freezes solid, expands, and splits — and a split bottle is a new bottle.
Frozen means stop pressing
Ice in the bottle is not a blockage you can push through. The pump is a small electric motor that stalls almost instantly when it meets a solid, and a stalled motor draws far more current than a running one. What happens next is either a blown fuse — the cheap outcome, and a reasonable design — or a burnt motor, which is not. The noise to listen for is nothing at all: if pressing the stalk produces no whirr, the fluid is solid or the pump has already gone. Thaw it properly, with the engine's own warmth and time, or park somewhere above freezing; do not pour hot water into a frozen bottle, because the thermal shock can crack it just as effectively as the ice.