1The color red — covering everything from pure red to deeper crimson and scarlet shades, the color of blood, sunsets, apples, and stop signals. Used as a noun and attributively with の; pairs with the i-adjective 赤い as the most basic Japanese color word.
私の好きな色は赤です。
My favorite color is red.
信号機の三色は赤・黄・青と決まっている。
The three colors on a traffic light are fixed: red, yellow, and green.
子どもが画用紙に赤で大きな丸を描いた。
The child drew a big circle on the drawing paper in red.
夕焼けの空は深い赤に染まっていた。
The sunset sky was dyed a deep red.
この口紅の赤は少し落ち着いた色合いで、大人っぽく見える。
The red of this lipstick has a slightly muted shade and looks mature.
2Red in the broader, traditional Japanese sense — not just pure red but the whole family of reddish hues, including pink, orange, vermilion, and reddish browns. Mostly seen in discussions of traditional color terminology, kimono, and crafts.
古い色名では、ピンクや朱色も広く「赤」のうちに含まれていた。
In older color terminology, even pink and vermilion were broadly counted as a kind of "red."
和服の世界では、橙や赤茶も「赤」の系統として扱われる。
In the world of kimono, oranges and reddish browns are treated as part of the "red" family.
3Slang term for a communist or someone with leftist sympathies — historically used in pre-war and Cold-War-era political contexts and often carrying a dismissive or pejorative tone. When written, it is usually rendered in katakana as アカ.
戦前は労働運動に関わるだけで「アカ」と呼ばれ、警察に目をつけられた。
Before the war, just being involved in the labor movement was enough to get someone called "a Red" and put on the police's radar.
政治的な意見が違うからといって、相手をアカ呼ばわりするのは乱暴だ。
Branding someone "a commie" just because you disagree with them politically is crude.
祖父の世代では、組合活動をする人をすぐにアカ扱いする風潮があった。
In my grandfather's generation, anyone involved in union activity was quickly written off as "a Red."
4Short for 赤信号 (red traffic light). Used in driving contexts in patterns like 信号が赤 (the light is red), 赤になる (turn red), and 赤を無視する (run a red).
信号が赤になったので、そこで止まってください。
The light has turned red, so please stop there.
赤を無視して交差点に進入した車が、事故を起こした。
A car ran the red and entered the intersection, causing an accident.
急いでいたが、赤だったので止まらざるを得なかった。
I was in a hurry, but the light was red, so I had to stop.
5Short for 赤字 — a financial loss / being "in the red" (expenses exceeding income). Also refers to red-pen corrections on a test or manuscript, used in expressions like 赤を入れる (mark up in red) and 赤が多い (lots of red marks).
今期の決算は三年連続の赤で、社内の空気は重い。
This term's results are in the red for the third year in a row, and the mood in the office is heavy.
返ってきた答案には、先生の赤がびっしり入っていた。
When the test came back, it was packed with the teacher's red corrections.
原稿に赤を入れる前に、まず通読して全体の流れをつかむ。
Before marking up the draft in red, I read it through once to get a feel for the overall flow.
6Used in fixed expressions of the form 赤の〜 / 赤〜 to mean "complete, total, utter." Typical examples: 赤の他人 (a complete stranger), 赤恥 (utter humiliation in front of others). Not productive — it appears only inside set idiomatic compounds, not as a free intensifier.
あの二人は親子のように見えるが、実は赤の他人だ。
Those two look like parent and child, but they're actually total strangers.
大勢の前で名前を間違えてしまい、赤恥をかいた。
I got the name wrong in front of a crowd and was utterly humiliated.
7An old abbreviation for copper (銅, traditionally read あかがね) or copper-made objects. Essentially absent from modern Japanese; survives only in traditional crafts, classical texts, and certain dialects.
古い茶釜には赤（あか）で作られた高価なものもあった。
Among old tea kettles there were valuable ones made of copper.
8In hanafuda (Japanese flower cards), an abbreviation for the red 5-point ribbon cards. Collecting all three (from the pine, plum, and cherry blossom months) forms the scoring combination 赤短 (akatan).
花札では松・梅・桜の赤を三枚そろえると「赤短」という役になる。
In hanafuda, collecting the three red ribbon cards from the pine, plum, and cherry months makes the scoring hand called 赤短.