One way to hold discoloration: colour quietly leaving. Nobody spilled anything and nobody did it on purpose — sunlight, damp, smoke or years simply pull the proper colour out of a surface and leave a wrong one behind. The word can point at the leaving, or at the patch it left. Which one you mean shows up in the grammar: no a for the change, a for the mark.
From the word’s history
Wiktionary records discoloration as discolor + -ation. The parts are visible: dis- reverses, color is the colour, -ation turns the action into a thing you can name.
字源資料把 discoloration 記成 discolor 加 -ation。三塊都看得見：dis- 表示去掉或相反，color 是顏色，-ation 把動作變成一個可以稱呼的名詞。所以字面就是「把顏色去掉這件事」，正好對上兩個意思共用的畫面。
Wiktionary/kaikki extract supplied in this evidence packet (source_ref kaikki:3f6f69a190036ae4:637061833): 'From discolor + -ation.'
登入就可以看每一支怎麼連回核心畫面。