As I understand, the emojis that the OP is talking about are UNICODE emojis.
Unfortunately, at this time the Google Slides presentation mode doesn't display them in full color, so instead of using UNICODE emojis, use images. For that, you could take a screenshot and snip out the emojis.
UNICODE emojis are pictorial representation of concepts that could be used inline with text and they not have to look the same (1,2).
Google Docs, Google Drawings and Google Slides allow the insertion of UNICODE emojis among other symbols through the menu
Insert > Special Character..., by copying and pasting or by pressing certain key combinations, but not all the special characters are available for all combinations of operating systems and browsers, in such cases a "tofu character" (an empty box) will be shown instead. But even if the special character is being displayed correctly and in full color in Google Slides edition mode, it could not being displayed in the same way in presentation mode.
I did some tests using a Chromebook with Chrome OS Version 48.0.2564.48 beta.
Test 1: I created a presentation with one blank slide including only a text block to hold the
UNICODE emoji. It was rendered in black & white. I took a look into the source of the page using the Chrome Developer tools and found that the pizza slice was rendered by using svg > path html tags and being drawn. Then I look to the code of the pizza slice in the presentation in edit mode, and it also was rendered by using svg > path html tags and being drawn. The attributes were slightly different.
Test 2: I added alphabet characters, they also were rendered by using svg > path html tags and being drawn.
After researching a bit today I found an article (3) where there are compared the use of icon fonts Vs. inline SVG and another where the use of inline SVG is explained including how to apply color to UNICODE emojis. I think that the Google Slide developers should read these articles :)