Pattern that matches colors

Question

Suppose I'm writing a function that takes a color as a parameter; for example:

drawShape[color_] := Graphics[{Style[Disk[], color]}];

But if the caller inputs an invalid color, bad things happen:

So I want to use a pattern to define drawShape only for values that are actually colors. Conceptually,

drawShape[color_Color] := ...

The problem is that unlike (say) Lists, Integers, Reals, Complexes, or Graphicses, color objects do not share a Color head. That is,

In[1]:= Red // Head
Out[1]= RGBColor
In[2]:= Hue[0.5] // Head
Out[2]= Hue
In[3]:= GrayLevel[0.5] // Head
Out[3]= GrayLevel
In[4]:= CMYKColor[0, 1, 1, 1/2] // Head
Out[4]= CMYKColor
In[4]:= Opacity[0.5, Purple] // Head
Out[4]= Opacity
In[5]:= Transparent // Head
Out[5]= GrayLevel

So that won't work. I also don't see any ColorQ function, with which I could write drawShape[color_ ? ColorQ] := ....

How can I write a pattern that matches any valid color object? Is there a more robust way than just testing for each of these heads?

Answer 1

Original method

colorQ = Quiet @ Check[Blend @ {#, Red}; True, False] &;

colorQ /@ {Red, Hue[0.5], GrayLevel[0.5], CMYKColor[0, 1, 1, 1/2], Opacity[0.5, Purple]}

{True, True, True, True, True}

colorQ /@ {17, 1.3, Pi, "not a color", {1, 2, 3}, Hue["bad arg"]}

{False, False, False, False, False, False}

You would use: drawShape[color_?colorQ] := . . .

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Inspired by kguler's comment this might also be formulated as:

colorQ = Quiet[Head @ Darker @ # =!= Darker] &;

Or:

colorQ = FreeQ[Quiet @ Darker @ #, Darker] &;

Edit: Darker works on entire Image and Graphics objects and therefore the two forms immediately above will incorrectly return True in these cases. Blend solution is still valid.

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Version 10 update and analysis

In version 10 there is a built-in function for this: ColorQ

ColorQ[color] yields True if color is a valid color directive and False otherwise.

A bit of spelunking reveals that the inner definition of this function is (contexts stripped for clarity):

iColorQ[args_?(ColorDirectiveQ[Head[#1[[1]]]] &), opts_] := 
  NumberQ[Quiet[ToColor[args[[1]], XYZColor][[1]]]]

This is very similar to my own method, however the inner definition of ColorDirectiveQ omits Opacity:

iColorDirectiveQ[args_, opts_] := 
 TrueQ[Quiet[
   MatchQ[args[[1]], 
    GrayLevel | RGBColor | CMYKColor | Hue | XYZColor | LUVColor | LABColor | LCHColor]]]

This means that the function will return False for e.g. Opacity[0.5, Purple] where mine returns True.

Answer 2

I think that the following catches all color heads.

Clear[test];
test[x : (_CMYKColor | _Hue | _RGBColor)] := "good"
test[else_] := "bad"

Let's try it

test[Red]
(* Out: "good" *)

test[red]
(* Out: "bad" *)