Set the Style in the Subtitle File Itself.

For SRT files, the options for the subtitle styles are limited.

• Basic styling can be set using HTML markups like <b>, <i>, <u>, and <font>.

subtitle.srt

1
00:00:0,000 --> 00:00:2,000
This is the <b><i>first</i></b> sentence
 
2
00:00:2,000 --> 00:00:4,000 
This is the second sentence
 
3
00:00:4,000 --> 00:00:7,000
This is the third sentence
 
4
00:00:7,000 --> 00:00:10,000
This is the forth sentence

For ASS files, there are many more options for the subtitle styles.

These are the styles options that can be set in an ASS file:

[V4+ Styles]
Format: Name, Fontname, Fontsize, PrimaryColour, SecondaryColour, OutlineColour, BackColour, Bold, Italic, Underline, StrikeOut, ScaleX, ScaleY, Spacing, Angle, BorderStyle, Outline, Shadow, Alignment, MarginL, MarginR, MarginV, Encoding
Style: Default,Arial,16,&Hffffff,&Hffffff,&H0,&H0,0,0,0,0,100,100,0,0,1,1,0,2,10,10,10,0

To change the font style to Futura and font color to “&HFF00” (green):

subtitle.ass

[Script Info]
; Script generated by FFmpeg/Lavc60.31.102
ScriptType: v4.00+
PlayResX: 384
PlayResY: 288
ScaledBorderAndShadow: yes
YCbCr Matrix: None
 
[V4+ Styles]
Format: Name, Fontname, Fontsize, PrimaryColour, SecondaryColour, OutlineColour, BackColour, Bold, Italic, Underline, StrikeOut, ScaleX, ScaleY, Spacing, Angle, BorderStyle, Outline, Shadow, Alignment, MarginL, MarginR, MarginV, Encoding
Style: Default,Futura,16,&Hff00,&Hffffff,&H0,&H0,0,0,0,0,100,100,0,0,1,1,0,2,10,10,10,1
 
[Events]
Format: Layer, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text
Dialogue: 0,0:00:00.00,0:00:02.00,Default,,0,0,0,,This is the first sentence
Dialogue: 0,0:00:02.00,0:00:04.00,Default,,0,0,0,,This is the second sentence
Dialogue: 0,0:00:04.00,0:00:07.00,Default,,0,0,0,,This is the third sentence
Dialogue: 0,0:00:07.00,0:00:10.00,Default,,0,0,0,,This is the forth sentence