[StainlessS]

Is it (in the main) perhaps down to a part of the 'pluge' (think thats what its called) being @ luma value 7 [EDIT: and 242] (ie non TV range).

A=ColorBars(pixel_type="YV12").BicubicResize(480,360)
B=A.ColorYUV(levels="TV->PC")
C=B.ColorYUV(levels="PC->TV")
A=A.ColorYUV(analyze=true)
B=B.ColorYUV(analyze=true)
C=C.ColorYUV(analyze=true)
StackVertical(A,B,C)

First image minimum luma is 7.




EDIT: Did not notice this line,

because ColorBars() has some strange levels

that would be the pluge 7 [EDIT: and 242] thing I think.
But even with pluge fixed with eg limiter, does exhibit strange behaviour as pin pointed by Orum.

ie like this

function s(clip c) {
	return c.ColorYUV(levels="TV->PC").ColorYUV(levels="PC->TV")
}

# Initial clip calls s() once because ColorBars() has some strange levels

ColorBars(pixel_type="YV12").Limiter
Interleave(last, last.s().s().s().s().s().s().s().s().s().s().s().s().s().s().s().s())
ColorYUV(analyze=true)

Still flickers a helluva lot.

EDIT:
Limiter dont work as required, mod to Levels [EDIT: This is rubbish, dont know why I thought Limiter did not work, it does]

function s(clip c) { return c.ColorYUV(levels="TV->PC").ColorYUV(levels="PC->TV") }

ColorBars(pixel_type="YV12").Bicubicresize(480,360)
Levels(16,1.0,235,16,235,coring=false)  # Limiter dont work as required, results in luma Min = 10, not 16, so use Levels
A=Last
B=last.s.s.s.s.s.s.s.s.s.s.s.s.s.s.s
StackVertical(A.ColorYUV(analyze=true),B.ColorYUV(analyze=true))

EDIT: Only 1 iteration

function s(clip c) { return c.ColorYUV(levels="TV->PC").ColorYUV(levels="PC->TV") }

ColorBars(pixel_type="YV12").Bicubicresize(480,360)
Levels(16,1.0,235,16,235,coring=false)  # Limiter dont work as required, results in luma Min = 10, not 16, so use Levels
A=Last
B=last.s                                         #.s.s.s.s.s.s.s.s.s.s.s.s.s.s
StackVertical(A.ColorYUV(analyze=true),B.ColorYUV(analyze=true))