docs: Document rule.color.merge

ChangeLog

@@ -174,6 +174,8 @@
   task is created.
 - New 'unwait' verbosity token generates a notification message when a waiting
   task becomes visible.
+- New 'rule.color.merge=yes|no' configuration option, that can optionally
+  turn off color blending engine.
 
 ------ current release ---------------------------
 

NEWS

@@ -25,6 +25,9 @@ New configuration options in Taskwarrior 2.4.5
     generated.
   - The new 'unwait' verbosity token generates a message when a waiting task
     becomes visible.
+  - The new 'rule.color.merge' configuration option can disable merging of colors
+    produced by different color rules. Use if your color scheme produces
+    unpleasing foreground and background combinations.
 
 Newly deprecated features in Taskwarrior 2.4.5
 

doc/man/task-color.5.in

@@ -240,6 +240,9 @@ It is possible to create a very colorful mix of rules.  With 256-color support,
 those colors can be made subtle, and complementary, but without care, this can
 be a visual mess.  Beware!
 
+In such cases, consider using the 'rule.color.merge=no' option to disable the
+color blending.
+
 The precedence for the color rules is determined by the configuration
 variable 'rule.precedence.color', which by default contains:
 

doc/man/taskrc.5.in

@@ -881,6 +881,22 @@ nothing, for example:
 .RE
 .RE
 
+.RS
+By default, colors produced by rules blend. This has the advantage of
+conveying additional information, by producing combinations that are not used
+by any particular rule directly.
+
+However, color blending can produce highlighting combinations which are not
+desired. In such cases, use the following option to disable this behaviour:
+.RE
+
+.TP
+.B rule.color.merge=yes
+Can be "yes" or "no". When "no", disables merging of colors produced by
+different color rules. Use if your color scheme produces unpleasing
+foreground and background combinations.
+
+
 See the task-color(5) man pages for color details.
 .RE