Figure 1: use more accessible colors

[spoeschel]

The green/orange colors in figure 1 (§ 3.4) are a bit difficult to distinguish for people with red-green colour blindness (like me). E.g. the following colors could be used here, changing orange to red (source):

• #50FF50
• #FF5F5F

[gkatsev]

This makes sense. I'll try and make the change when I next get some time to work on WebVTT.