This rare kanji has no clear standalone meaning. Treat its meaning as uncertain.
赩 combines 赤 (red) and 色 (color), suggesting a deep red hue, but the exact historical development is uncertain.
The left side 赤 means red, and the right side 色 means color. Together they suggest a vivid red color, like a deep crimson.
For キョク, imagine a deep red kyoku (a made-up word for a red pigment) being mixed: kyoku -> キョク, and the red color spreads.