The whole poster itself looks like a vector drawing. The best way to "simulate" vector drawing in a raster program (like GIMP) is to use separate layers for each different color you need, paint each layer evenly with a single color, and then use layer masks to block out the parts of each layer that you don't want.
You can have a problem if you want to scale the image after it's completed though; vector formats don't lose quality when they are scaled.
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Now then.
So you wanna make fonts?
----Fontlab is probably the premier editing utility now, it will edit pretty much any kind of vector font. It costs a lot, a few hundred US dollars. It can handle a lot of formats.
----There's another small one named Font Creator that will only edit truetype fonts, and it will not perform hinting. But it does work. "Hinting" means that if you use your edited fonts in something like MS WOrd, they will look horribly blurry--but most graphic programs can still render them ewll enough, and they will print100% perfect.
The programs I have are a few years old now, but anyway:
---Macromedia used to offer a great one called Fontographer. It hasn't been sold for 5-6 years now, and wasn't updated at all for a couple years before that. The last version released will not even run on a PC with more than 512 megs of RAM. I just got a laptop with 512 megs total, so I can use it again!!! (I'm so hoppy!) It can handle a lot of font formats as well. Some people have gotten it to work on some computers various ways, but I was never successful.
---And no doubt there are other programs out there also, but when I was buying, Fontlab and Fontographer were the Big Two. Font Creator is "just a cheapy" that I initially found and kinda liked.
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---OpenType is a newer format (you may have seen in WindowsXP), but not one that you really need. Adobe and Microsoft at least both have a program that will create/edit OpenTye fonts, but I don't really know anything about these programs except that they cost a buttload of money. The advantages of OpenType are not significant as far as you are concerned; the old-fashioned truetype format will let you put any -vector- picture you want into any character code. You can create a totally-new font from scratch, but it is easiest to just edit an existing freeware font file that only has the basic alphanumeric/punctuation glyphs. ....The main thing that OpenType can do is store a sort of programming code inside the font, that is based on applied conditions. For example: some languages change the form of a preceeding character, based on whatever character is typed following it. OpenType stores that conditional data and so programs can perform these changes automatically, very easily. Certain combinations of character forms simply aren't used in these languages, and OpenType data can point to an alternate combination form that is proper.
For English this ability is not important, as English doesn't use this writing convention (I forget which languages do, it's Arab or Chinese or somethin, some of them scribble-languages that nobody uses). There are other things this data storage can be used for, but that is the main one intended.
TrueType doesn't do that; tryetype just links a vector drawing with a keycode. There is no information stored in the truetype format that would allow such conditional analysis.
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Truetype will not print "multicolor fonts" however. Some graphics programs have ways to set the outline and fill as different colors, but the colors are selected when you actually use the font, they are not "set" in the font file. Font files do not store color data at all.
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Aside from that, I can't help you.