Support specifying hex colors where no color name is needed

This was requested by designer2k2 in #219 for bgcolor usage.
It has also been discussed in #135.

The input validation is more detailed to help the user identifying
and locating invalid values. The wire color padding is now done on
the output to cover different input alternatives.

docs/syntax.md

@@ -399,6 +399,9 @@ The following colors are understood:
 <!-- color list generated with a helper script: -->
 <!-- https://gist.github.com/formatc1702/3c93fb4c5e392364899283f78672b952 -->
 
+Unless the color also is displayed as a non-hexadecimal text in the diagram,
+it is possible to specify colors as hexadecimal RGB values, e.g. `#112233`.
+
 ## Cable color codes
 
 Supported color codes:

src/wireviz/wv_colors.py

@@ -107,23 +107,37 @@
 
 color_default = '#ffffff'
 
+_hex_digits = set('0123456789abcdefABCDEF')
 
 def get_color_hex(input, pad=False):
+    """Return list of hex colors from either a string of color names or :-separated hex colors."""
     if input is None or input == '':
         return [color_default]
+    elif input[0] == '#':  # Hex color(s)
+        output = input.split(':')
+        for i, c in enumerate(output):
+            if c[0] != '#' or not all(d in _hex_digits for d in c[1:]):
+                if c != input:
+                    c += f' in input: {input}'
+                print(f'Invalid hex color: {c}')
+                output[i] = color_default
+    else:  # Color name(s)
+        def lookup(c: str) -> str:
+            try:
+                return _color_hex[c]
+            except KeyError:
+                if c != input:
+                    c += f' in input: {input}'
+                print(f'Unknown color name: {c}')
+                return color_default
 
-    if len(input) == 4:  # give wires with EXACTLY 2 colors that striped/banded look
-        padded = input + input[:2]
-    elif pad and len(input) == 2: # hacky style fix: give single color wires a triple-up so that wires are the same size
-        padded = input + input + input
-    else:
-        padded = input
+        output = [lookup(input[i:i + 2]) for i in range(0, len(input), 2)]
+
+    if len(output) == 2:  # Give wires with EXACTLY 2 colors that striped look.
+        output += output[:1]
+    elif pad and len(output) == 1:  # Hacky style fix: Give single color wires
+        output *= 3                 # a triple-up so that wires are the same size.
 
-    try:
-        output = [_color_hex[padded[i:i + 2]] for i in range(0, len(padded), 2)]
-    except KeyError:
-        print(f'Unknown color specified: {input}')
-        output = [color_default]
     return output