Colourant (3)
I believe the answer is:
dye
<span class="explanation-format__ClueExplanation">'<span class="explanation-format__original">colourant</span>' is the definition.<br/ ><span class="explanation-format__explanation">(I've seen this before)</span><br/ ><br/ >This is the entire clue.<br/ ></span>
(Other definitions for dye that I've seen before include "Colour by dipping in liquid" , "Colorant" , "Tint" , "Colourant" , "Staining material" .)