Use the Custom Video Display feature to help with color management
A colorspace defines how colors in a video are encoded (color primaries, gamma, transfer functions, white point). When two pieces of content are blended together, at least one element must undergo colorspace conversion so that their color models match. Mismatches can cause unpredictable results, such as an eyedropper tool that appears to choose the wrong color.
SyncSketch features colorspace conversion as part of its Custom Video Display feature. Make sure that it is checked so that colors behave predictably.
The option can be found in the Review Player, in the gear menu at the lower right. (Note: This function is currently unavailable on mobile devices.)
A little more detail
Color managed systems rely on metadata to understand how the colors in a piece of video are encoded. SyncSketch sketches and the sketching tools are always operating in the sRGB colorspace. If the content you're uploading is tagged as sRGB (with the color profile tag 1-13-1), the colors should match without conversion, and enabling this feature may not appear to have any effect.
However, if your content is encoded as Rec.709 – really, with any other color profile, like HD (1-1-1) – the Custom Video Display feature should be enabled to avoid nasty surprises.
Here's an example of the File Info window of the Mac Finder revealing how the colorspace is tagged for a piece of video.