To change the colour of a shape in a Synthesia video:
Navigate to the chosen video.
Select the Shape on a scene.
Click on the Fill option in the right-hand menu.
Select the new colour for your shape.
Alternatively - use the Gradient option for your shape assets.
To change the colour of a text (titles, subtitles, and paragraphs) in a Synthesia video:
Navigate to the chosen video.
Select the text on a scene.
Click on the Font colour option in the right-hand menu.
Select the new colour for your text.
Notes:
Using the Change all button in the Fill section makes all shapes/text with this colour get the new one.