custom shader color seems strange

Hello, I’m trying to make material for UI background.
First, I just map UV to the output color and get

looks like right but too light.I try the same thing at webGL, I got

obviously, gl has the darker scene and reasonable on mathematical .
later I tryed draw shape and color both WebGL and ue4 HLSL.
I found the position caculation is right(shape did completely well represented) but
final color was be lighted. Like this HSV cycle scene in Gl:

I got custom shader:

I don’t learn hlsl for long, so I’m not sure what’s wrong with custom shader coord(I useTexcoord[0] as init UV).
can anyone give me some adivce about this strange color calculate result? or basic right coord UV input?