Use the setal capability as well as (tmux's) Setulc.

tmux.1

@@ -5897,8 +5897,8 @@ Set a styled underscore.
 The single parameter is one of: 0 for no underscore, 1 for normal
 underscore, 2 for double underscore, 3 for curly underscore, 4 for dotted
 underscore and 5 for dashed underscore.
-.It Em \&Setulc
+.It Em \&Setulc , \&ol
-Set the underscore colour.
+Set the underscore colour or reset to the default.
 The argument is (red * 65536) + (green * 256) + blue where each is between 0
 and 255.
 .It Em \&Ss , Se

tmux.h

@@ -448,6 +448,7 @@ enum tty_code_code {
 	TTYC_KUP6,
 	TTYC_KUP7,
 	TTYC_MS,
+	TTYC_OL,
 	TTYC_OP,
 	TTYC_REV,
 	TTYC_RGB,
@@ -459,6 +460,7 @@ enum tty_code_code {
 	TTYC_SE,
 	TTYC_SETAB,
 	TTYC_SETAF,
+	TTYC_SETAL,
 	TTYC_SETRGBB,
 	TTYC_SETRGBF,
 	TTYC_SETULC,

tty-features.c

@@ -112,6 +112,7 @@ static const struct tty_feature tty_feature_overline = {
 static const char *tty_feature_usstyle_capabilities[] = {
 	"Smulx=\\E[4::%p1%dm",
 	"Setulc=\\E[58::2::%p1%{65536}%/%d::%p1%{256}%/%{255}%&%d::%p1%{255}%&%d%;m",
+	"ol=\\E[59m",
 	NULL
 };
 static const struct tty_feature tty_feature_usstyle = {
@@ -336,7 +337,7 @@ tty_default_features(int *feat, const char *name, u_int version)
 	"256,RGB,bpaste,clipboard,strikethrough,title"
 		{ .name = "mintty",
 		  .features = TTY_FEATURES_BASE_MODERN_XTERM
-			      ",ccolour,cstyle,extkeys,margins,overline"
+			      ",ccolour,cstyle,extkeys,margins,overline,usstyle"
 		},
 		{ .name = "tmux",
 		  .features = TTY_FEATURES_BASE_MODERN_XTERM

tty-term.c

@@ -245,6 +245,7 @@ static const struct tty_term_code_entry tty_term_codes[] = {
 	[TTYC_KUP6] = { TTYCODE_STRING, "kUP6" },
 	[TTYC_KUP7] = { TTYCODE_STRING, "kUP7" },
 	[TTYC_MS] = { TTYCODE_STRING, "Ms" },
+	[TTYC_OL] = { TTYCODE_STRING, "ol" },
 	[TTYC_OP] = { TTYCODE_STRING, "op" },
 	[TTYC_REV] = { TTYCODE_STRING, "rev" },
 	[TTYC_RGB] = { TTYCODE_FLAG, "RGB" },
@@ -255,6 +256,7 @@ static const struct tty_term_code_entry tty_term_codes[] = {
 	[TTYC_RMKX] = { TTYCODE_STRING, "rmkx" },
 	[TTYC_SETAB] = { TTYCODE_STRING, "setab" },
 	[TTYC_SETAF] = { TTYCODE_STRING, "setaf" },
+	[TTYC_SETAL] = { TTYCODE_STRING, "setal" },
 	[TTYC_SETRGBB] = { TTYCODE_STRING, "setrgbb" },
 	[TTYC_SETRGBF] = { TTYCODE_STRING, "setrgbf" },
 	[TTYC_SETULC] = { TTYCODE_STRING, "Setulc" },

tty.c

@@ -2543,7 +2543,7 @@ tty_colours_fg(struct tty *tty, const struct grid_cell *gc)
 	/* Is this a 24-bit or 256-colour colour? */
 	if (gc->fg & COLOUR_FLAG_RGB || gc->fg & COLOUR_FLAG_256) {
 		if (tty_try_colour(tty, gc->fg, "38") == 0)
-			goto save_fg;
+			goto save;
 		/* Should not get here, already converted in tty_check_fg. */
 		return;
 	}
@@ -2555,13 +2555,13 @@ tty_colours_fg(struct tty *tty, const struct grid_cell *gc)
 			tty_puts(tty, s);
 		} else
 			tty_putcode1(tty, TTYC_SETAF, gc->fg - 90 + 8);
-		goto save_fg;
+		goto save;
 	}
 
 	/* Otherwise set the foreground colour. */
 	tty_putcode1(tty, TTYC_SETAF, gc->fg);
 
-save_fg:
+save:
 	/* Save the new values in the terminal current cell. */
 	tc->fg = gc->fg;
 }
@@ -2575,7 +2575,7 @@ tty_colours_bg(struct tty *tty, const struct grid_cell *gc)
 	/* Is this a 24-bit or 256-colour colour? */
 	if (gc->bg & COLOUR_FLAG_RGB || gc->bg & COLOUR_FLAG_256) {
 		if (tty_try_colour(tty, gc->bg, "48") == 0)
-			goto save_bg;
+			goto save;
 		/* Should not get here, already converted in tty_check_bg. */
 		return;
 	}
@@ -2587,13 +2587,13 @@ tty_colours_bg(struct tty *tty, const struct grid_cell *gc)
 			tty_puts(tty, s);
 		} else
 			tty_putcode1(tty, TTYC_SETAB, gc->bg - 90 + 8);
-		goto save_bg;
+		goto save;
 	}
 
 	/* Otherwise set the background colour. */
 	tty_putcode1(tty, TTYC_SETAB, gc->bg);
 
-save_bg:
+save:
 	/* Save the new values in the terminal current cell. */
 	tc->bg = gc->bg;
 }
@@ -2605,20 +2605,34 @@ tty_colours_us(struct tty *tty, const struct grid_cell *gc)
 	u_int			 c;
 	u_char			 r, g, b;
 
+	/* Clear underline colour. */
+	if (gc->us == 0) {
+		tty_putcode(tty, TTYC_OL);
+		goto save;
+	}
+
 	/* Must be an RGB colour - this should never happen. */
 	if (~gc->us & COLOUR_FLAG_RGB)
 		return;
 
 	/*
-	 * Setulc follows the ncurses(3) one argument "direct colour"
+	 * Setulc and setal follows the ncurses(3) one argument "direct colour"
 	 * capability format. Calculate the colour value.
 	 */
 	colour_split_rgb(gc->us, &r, &g, &b);
 	c = (65536 * r) + (256 * g) + b;
 
-	/* Write the colour. */
+	/*
+	 * Write the colour. Only use setal if the RGB flag is set because the
+	 * non-RGB version may be wrong.
+	 */
+	if (tty_term_has(tty->term, TTYC_SETULC))
 		tty_putcode1(tty, TTYC_SETULC, c);
+	else if (tty_term_has(tty->term, TTYC_SETAL) &&
+	    tty_term_has(tty->term, TTYC_RGB))
+		tty_putcode1(tty, TTYC_SETAL, c);
 
+save:
 	/* Save the new values in the terminal current cell. */
 	tc->us = gc->us;
 }