The closest to a 'standard' as far as I know is SFont, a convention for storing fonts in bitmaps (eg PNG, GIF). (S standing for
SDL here)
Here's an example font I made:
This is using GrafX2's SFont variant, which I don't really know how it differs from others.
But the basic idea of SFont is:
* Begin with a blank (index #0 filled) canvas.
* All characters on one horizontal line in ascii order, starting with character #33 : '!'
* Minimum of 1 pixel gap with a 'spacer' color (#1 is used here, but whatever color is in the top left corner of the image can be determined to be the spacer color). Larger gaps are okay.
* Characters are variable width but fixed height, to simplify rendering.
* Characters are a minimum of 1px wide (there is no way to notate 0px-wide characters)
* Characters can contain any palette indices except the spacer color. These indices are interpreted normally (ie. each color remains distinct, you can alter palette entries when rendering to get different tinted characters, etc.)
* No kerning or line spacing information is included.
* No specific UTF-8 / Unicode support. Accented characters etc are normally done using a particular codepage (== extension of the ASCII standard). The fonts that come with GrafX2 include some examples of support for accented characters.
This will allow you to use your font in full color in GrafX2 or any game that uses SDL for rendering. It's a simple enough format that any half-decent programmer should be able to hack together something that handles it in relatively little code.
(BTW: GrafX2 does not make palette-swapping the font super easy, but you can do it by opening the font file in the spare page, editing the palette, and re-saving (GrafX2 uses the updated font next time you use text tool). Or you can just generate 'multiple' font files that are simply different palette swaps as you need.)
As far as I know, no 'high end' programs like PhotoShop or GIMP support non-monochrome bitmap fonts. There is no 'proper' standard yet -- SVG fonts exist and support similar features to pixel fonts as well as the features of vector fonts, but support for SVG fonts is currently poor.
EDIT:
http://blog.symbolset.com/multicolor-fonts has some interesting information on this subject. Apparently there -is- some system support for multicolor fonts, but it's not consistent. FreeType supports Google's format and Apple's format, so Linux support for both of those is probably available.; Adobe+Mozilla's format is most powerful but is only supported
by Firefox; Microsoft's format, as you might expect, is only supported on Windows; and MPEG are working on creating an open multicolor font standard unifying ideas from all these different implementations.
How to actually author this kind of bitmap font is still a mystery to me. I'm also unsure whether these font formats are intended for general use or for web only use.
EDIT2:
Apparently Type3 fonts (a subset of OpenType, which is what TTF uses) support multicolor glyphs. It's unclear whether you can easily render Type3 OpenType fonts to screen or not.