In 0.24.0:
1. when saturation is 1, the background image is original image (very light).
2. when saturation is less 1 but near 1, the image is black (dark).
3. the saturation level is reversed compared with vte <= 0.23.5

In 0.23.5 and before:
1. when saturation is near 1, the background image is light.
2. when saturation is near 0, the background image is dark.

The browser I am using has problem to attach the patch, so I paste the very simple patch here:

--- vte-0.24.0.orig/src/vtebg.c
+++ vte-0.24.0/src/vtebg.c
@@ -505,7 +505,7 @@ vte_bg_get_surface(VteBg *bg,
 				       tint->red / 65535.,
 				       tint->green / 65535.,
 				       tint->blue / 65535.,
-				       saturation);
+				       1 - saturation);
 		cairo_set_operator (cr, CAIRO_OPERATOR_OVER);
 		cairo_paint (cr);
 	}

The patch will fix the problem for 0.24.0