Oftentimes, when users propose a UI change (in the meta sites, for example, but not limited to) they often create a mockup using a screenshot of the existing UI with elements modified.
I have seen some samples (I regret that I searched and I couldn't come up with one) that have introduced new text into the picture. I am wondering what software these users employed to match the fonts, and in some occasions scale them. I suppose they could have found samples of each letter and weaved them together, but my sense is that this isn't the case.
I am also amazed by the color matching, I'm assuming that a program even on the level of Paint can have an "eyedropper" tool to grab the color, but some of the samples that I've seen have used it more elaborately.
The simple answer to both of these questions may just be Photoshop/GIMP/etc (as I'm not an advanced user of any of those, I may not be aware of the features to do so), but are there any tools designed for the sole purpose of doing UI mockups?