Hmm. Which app did you use to display the subtitle?
I think it depends on how the app converts YUV to RGB.
SupTitle uses the following formula:
YUV to RGB Conversion
B = 1.164(Y - 16) + 2.018(U - 128)
G = 1.164(Y - 16) - 0.813(V - 128) - 0.391(U - 128)
R = 1.164(Y - 16) + 1.596(V - 128)
From here
http://www.fourcc.org/fccyvrgb.php
Which app created the sup file?