The basic answer is that the emoji are done using OpenType SVG fonts. Such fonts provide font definitions both in SVG format (basically SVG vector graphics) as well as in standard CFF or TrueType format for those applications that don't support SVG format fonts.
At this point, the PDF specification, including the relatively new ISO 32000 PDF 2.0 specification does not support the SVG format in fonts. Thus, if there is text in a PDF file referencing an OpenType SVG font (whether embedded or external), PDF viewers (including PDF native printers) will render the glyphs in that text in a single color using the CFF or TrueType definition. That accounts for your seeing the text as “black and white.” There is way to fix that.
There are workarounds that some applications and PDF creators use. One such workaround is to rasterize or vectorize any glyphs for which the SVG definition varies from the default monochrome CFF or TrueType definition. In that case, those “characters” are represented in the PDF file as raster or vector graphics. Alternatively, the PDF creator can produce and embed a Type 3 font into the PDF for such characters. (Type 3 fonts are essentially the equivalent of an arbitrary PostScript program, allowing for almost anything including color!)
The same issues also occur with so-called color fonts in which glyphs are in multiple colors.
Some applications (including some of the Adobe CC applications do apply one or another of the above hacks when exporting/saving PDF. Others don't, including the current Acrobat PDFMakers for Office.
- Dov
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