colorspace only matters when you have to deal with video splices... But, thanks for reminding me this... I just removed colorspace and automatically set colorspaces for the video clips splices
over here.
(and the only part I haven't bother checking is the audio portion of the video clips splices, so one will have to convert them manually -- but it's an easy task to convert though in my avisign function... I just never encountered problems with it though)
To splice in a video or part of the video clip in replacement of the video portion -- very much like overlay without the Alpha channel - only faster.
avisign( "sign.avi" ).sign( 100 , mode="splice" )
Sometimes you need to use splices because how AFX transfer blending modes work differently from AVISynth, so overlay will not always work.
Also, you can use v1b codec if you want to with a separate RGB and Alpha channel, defined... or jpg with a separate grayscale mask file.