Update color-4.vim

https://www.w3.org/TR/2020/WD-css-color-4-20201112/

test/test.css

@@ -83,7 +83,6 @@
 	display: hwb(120deg, 44%, 50%);
 	display: lab(1 2 3);
 	display: lch(4 5 6);
-	display: gray(50%);
 	display: color(swopc, 0 206 190 77);
 	display: device-cmyk(0, 81%, 81%, 30%);
 	display: relative-colorimetric;