add repeated washes of color
渲 centers on applying repeated washes of color, especially in painting or dyeing, to build up depth or tone.
渲 combines 氵 (water) with 宣, which likely contributes the sound and may relate to spreading or declaring, suggesting the idea of spreading color with water.
Water 氵 beside 宣: imagine a brush repeatedly dipping into water to spread and layer washes of color across a painting.
For セン, picture a painter adding a wash of color to a scene: scene -> セン, and each wash builds the scene.
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