Add JQ_COLORS env var for color config (fix #1252)

docs/content/3.manual/manual.yml

@@ -160,6 +160,9 @@ sections:
         terminal. You can force it to produce color even if writing to
         a pipe or a file using `-C`, and disable color with `-M`.
 
+        Colors can be configured with the `JQ_COLORS` environment
+        variable (see below).
+
       * `--ascii-output` / `-a`:
 
         jq usually outputs non-ASCII Unicode codepoints as UTF-8, even
@@ -3195,3 +3198,42 @@ sections:
           could then use to, for example, search for, download, and
           install missing dependencies.
 
+  - title: Colors
+    body: |
+
+      To configure alternative colors just set the `JQ_COLORS`
+      environment variable to colon-delimited list of partial terminal
+      escape sequences like `"1;31"`, in this order:
+
+        - color for `null`
+        - color for `false`
+        - color for `true`
+        - color for numbers
+        - color for strings
+        - color for arrays
+        - color for objects
+
+      The default color scheme is the same as setting
+      `"JQ_COLORS=1;30:0;39:0;39:0;39:0;32:1;39:1;39"`.
+
+      This is not a manual for VT100/ANSI escapes.  However, each of
+      these color specifications should consist of two numbers separated
+      by a semi-colon, where the first number is one of these:
+
+        - 1 (bright)
+        - 2 (dim)
+        - 4 (underscore)
+        - 5 (blink)
+        - 7 (reverse)
+        - 8 (hidden)
+
+      and the second is one of these:
+
+        - 30 (black)
+        - 31 (red)
+        - 32 (green)
+        - 33 (yellow)
+        - 34 (blue)
+        - 35 (magenta)
+        - 36 (cyan)
+        - 37 (white)