The .srt format does not officially contain font, color, size or location information : it only contains raw text and timing for each subtitle.
These parameters hence depend on the video player application you use to read them. For example, in Subs Factory, the yellow is used to differentiate the subtitles in case a second subtitles file is opened alongside the first (for this second file, subtitles will be blue). They will not have this color outside Subs Factory in another video player application, because their color will then depend on this other application's settings.
It is the same for the size. However, there is an option for the size to fit the video size or to be bigger for a better readability : see ergonomy preferences.
This said, there is an unofficial way to display some style effects: see "Formatting" paragraph of this Wikipedia page. These effects are correctly displayed in Subs Factory video, but the markup has to be entered as text. It is possible that their edition will be simplified in a future version of Subs Factory.
As this is not officially part of .srt, it may not be displayed correctly in every video player application.
Subs Factory is not able to burn subtitles into a video - that would need a complete reencoding of the video, which is a heavy operation. To do such a thing, take a look at FFmpegX : this application is complicated to install, but should do the work. On top of that, it is a shareware. Another older application, D-Vision, should also be able to do that.
However, are you sure you must burn the subtitles in ? Using .srt files is accepted by a lot of video players, including TV boxes. Even YouTube allows to upload those files. They have the huge advantage to be deactivable at anytime, and to remain modifiable.