信号は赤に変わった。

signal; signalling; signaling; traffic light; traffic signal

topic marker particle; indicates contrast with another option (stated or unstated); adds emphasis

red; crimson; scarlet; red-containing colour (e.g. brown, pink, orange); Red (i.e. communist); red light; red ink (i.e. in finance or proof-reading); (in) the red; complete; total; perfect; obvious; copper

at (place, time); in; on; during; to (direction, state); toward; into; for (purpose); because of (reason); for; with; by; from; as (i.e. in the role of); per; in; for; a (e.g. "once a month"); and; in addition to; if; although

to change; to be transformed; to be altered; to vary; to move to; to be different; to be uncommon; to be unusual