YTSubConverter

YouTube's built-in subtitle editor doesn't support styling of any kind. If you want formatting such as bold, italic and coloring, you need to upload a subtitle file instead. The site accepts a number of file formats such as RealText, WebVTT and TTML, but all of these come with their own limitations - and most importantly, none of them give access to the full array of features offered by the YouTube player. For that, you need to use a YouTube-specific format called SRV3 (also referred to as YTT for "YouTube Timed Text"). It supports the following:

• Bold/italic/underline
• Text coloring and transparency
• Background coloring and transparency (including hiding the background box completely)
• Outlines, glows and drop shadows
• Fonts and font sizes
• Positioning (place your subtitles anywhere on the video)
• Karaoke timing (make the syllables of a song lyric appear as they're sung)
• Vertical text
• Ruby text

YTSubConverter can produce this file format for you.