ImageMagick：带有文本图像的合成图像，结果图像将白色背景反转为黑色

I really need some help to understand the image channel problem, I used ImageMagick to gen a text image with jpg 300 dpi RGB.

I then merge this image on the top of a background image(jpg 300 dpi CMYK 8bits/Channel).

I find the text image is normal when open this by default/photoshop, however when I merge to the background image, it have become inverting the color tone, black and white.

can anybody tell me what's wrong with the image? please help I have tried couple of hours however still no answer I'm using ImageMagick 6.7.9-10

This is the command I ran:

convert -background white -fill black \
      -font 'Brandon_reg.otf' \
            -pointsize 9 -size 445x -density 300 -gravity center\
            caption:'Why this message become invert the black and white?' \
      msg.jpg"


convert background.jpg msg.jpg \
            -gravity center -composite output.jpg

please see this image