problem using Marx libvlc c# wrapper with 'vmem'&'subtitles'

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Hello guys
I have some problem using the Marx c# wrapper for libvlc.
I want to add the subtitle on the movies, but when the movie is rendered the subtitles always are in a fixed mode(e.g the color font is always black). I added this line in the argv ("--freetype-color=<string>") to have more colors, but the subtitle font color is still Black.
I also render the movies in "--vout", "vmem".
A strange result is this: when I render the movie in "--vout", "directx" instead of "--vout", "vmem" the problem is resolved.
Would you mind helping me guys?
Thank you in advance for your attention.