I have a black and white image, a scanned signature, where I want to change the colour of the black lines to blue (or red or something else). How do I do this? Preferably with freeware (I have Mac, Win7 and Linux, I prefer the Mac).

  • 1
    You may find this Q&A for Gimp helpful: graphicdesign.stackexchange.com/questions/21780/… (procedures will be very similar in other applications).
    – Takkat
    Jan 21, 2015 at 12:23
  • 1
    Is it genuinely black-and-white, not greyscale? This affects the answer significantly
    – e100
    Jan 21, 2015 at 15:06
  • @e100, I have different versions of it, 1-bit, 8-bit grayscale and 32-bit full colour.
    – d-b
    Jan 21, 2015 at 15:34

4 Answers 4


There are several ways to do this in GIMP, or in any other raster image editor (Photoshop, Krita, Paint.NET, etc.) implementing similar features. Here's one way to do it, demonstrated on a famous example:

  1. Scan and crop the signature:

    John Hancock's signature from the U.S. Declaration of Independence

  2. Convert it to grayscale, and adjust the levels to make the background pure white. If necessary, clean up any specks of dirt or other unwanted details:

    Hancock signature, grayscale and levels adjusted

  3. Pick a nice shade of dark blue (or whatever color you want), and create a new layer filled with that color. (If it comes out dark gray instead of blue, change the image to RGB mode and try again.) Move it over the signature layer, and change the layer blending mode to Screen:

    Screenshot of GIMP layers dialog, showing signature and color screen

  4. Admire the result:

    Hancock signature, colored blue

Of course, there are several other ways to do this, as well. For example, you could:

  • put the signature layer above the color layer, and change its blending mode to Screen instead;

  • apply the signature as a layer mask on the color layer;

  • use Color to Alpha to make the black areas of the signature transparent, then position it over the color layer;

  • use the Levels tool (or Curves) to adjust the blue channel, increasing the low end of the output range; or

  • just use the built-in Colorize tool.

I'm sure there are other methods I've left out, too.

All of these methods have their advantages and drawbacks. For example, the Levels / Curves and Colorize tools are quick to use, and give you a single colorized layer without having to merge layers, but they're not so convenient if you already have a specific color picked out, and they don't allow you to use more complicated textures instead of just plain solid colors.

On the other hand, the Curves tool (and also Levels, to some extent) allows non-linear color adjustments, which can let you simulate very dark inks (that look almost black when applied densely, but show some color on the edges and in thinner strokes) better. That requires a bit more skill to get right, though.

Meanwhile, the Layer Mask method gives you a blue-on-transparent layer instead of blue-on-white, which might be useful sometimes. (But do note that you can achieve a very similar effect by applying to Color to Alpha to the finished blue-on-white layer, anyway.)

  • 2
    Thank you for your detailed answer. I don't have enough reputation to upvote it yet but when I get that I will.
    – d-b
    Jan 22, 2015 at 19:49

Thanks to the inspiration from the comments and answers above I downloaded Graphic Converter (for Mac) and played around a bit. Using the following settings in the dialog Picture -> Brightness/Contrast/Hue the black line become blue

enter image description here

Graphic Converter is not freeware but there is a 30 day demo period and that was enough for me in this case.

  • This is fine if you don't need a specific colour value; I'd assumed you wanted to end up with an exact blue or red shade.
    – e100
    Jan 21, 2015 at 19:07
  • @e100, what I try to do is to fill in a pdf-form, "stamp" it with this scanned signature, print it and then snailmail it - and make the receiver believe the signature is handwritten. A different colour is a way to achieve this impression (they have returned the form when I previously used the black signature). But of course, I am interested in any additional information: so how do you pick an exact colour value?
    – d-b
    Jan 21, 2015 at 19:19
  • @user23122: Why not just sign the form by hand? Or are you trying to forge someone else's signature? If so, please do note that a) this is dishonest, and possibly illegal, and b) no matter what you do, the recipient can probably tell a printed signature from a handwritten one, if they look closely. Jan 22, 2015 at 19:59
  • Of course some smartass was gonna ask that question. Guess what, I am on long trip using satellite connection to write this. I want to sign a form and e-mail it to a friend who will print it and send it somewhere. Is that a valid reason or do you think I should go straight to jail?
    – d-b
    Jan 22, 2015 at 21:44
  • That does seem like a good reason, and may even legally qualify as a valid signature. (I'm not a lawyer, but AFAIK, the law in most places tends to follow the principle that if you call something your signature, it is your signature.) Still, my warning stands: if the place you're sending this to is nitpicky enough to have already rejected one form for an "invalid signature", there's no guarantee that they'll accept a re-colored one either. A printed picture of a signature just won't look quite the same as a handwritten one, at least not unless you use a really high-quality printer. Jan 23, 2015 at 0:08

Alternative, assuming a black and white document, like text.

  • Image -> Mode -> Indexed... -> Use Black and White (1 bit) palette -> Ok

  • Image -> Mode -> RGB

  • Colors -> Map -> Color Exchange... -> From Color -> Pick pure black (HTML notation ffffff) -> To Color -> Pick a blue shade


For that I will suggest that you use hue and saturation which will fulfill your requirement.

  • 1
    How? Please add some more information to your answer, so we know how to actually solve the Question's problem. Thanks!
    – Vincent
    Jan 21, 2015 at 13:43

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