Black/White Image with unsigned char is blue instead of black C++ - c++

Question

I'm reading an .img Gray Scale Image (512 byte header, 512 x 512 pixels x 8 bits per pixel).
There I'm determining a threshold to turn the Gray-Scale image into a binary image (black/white).
But when I assign 0 to the pixels, it turns out to be a light blue (#01fffd)?!?! When I use the value 2 it's almost black (#020100).
Can you explain me how this comes?
I read the image like this:
int width = 512;
int height = 512;
File* imageFilePointerIn = fopen("image.img", "r+b");
File* binaryImageFilePointerOut = fopen("imageOut.img",
"w+b");
unsigned char* header = new unsigned char[512];
unsigned char** imageData = new unsigned char*[width];
unsigned char** binaryImageArray = new unsigned char*[width];
fread(header, sizeof(char), 512, imageFilePointerIn);
for (int i = 0; i < width; i++) {
data[i] = new unsigned char[height];
fread(data[i], sizeof(char), height, imageFilePointerIn);
}
for (int i = 0; i < width; i++) {
binaryImageArray[i] = new unsigned char[height];
}
char unsigned thresholdBinaryImage = 128;
for (int i = 0; i < width; i++) {
for (int j = 0; j < height; j++) {
if (thresholdBinaryImage <= data[i][j]) {
binaryImageArray[i][j] = 255;
} else {
binaryImageArray[i][j] = 0;
}
}
}
fwrite(header, sizeof(char), headerSize, binaryImageFilePointerOut);
for (int i = 0; i < width; i++) {
fwrite(binaryImageArray[i], sizeof(char), height,
binaryImageFilePointerOut);
}
fclose(binaryImageFilePointerOut);
fclose(imageFilePointerIn);
I have no idea why colors could be displayed at all?
For displaying I transfer the .img file into a .ras file with a code snipped from the internet. But I don't think that the error could be there since I saw others using the same code for transforming .img Grayscales into .ras just for displaying without having colors.
Would be nice to get any suggestion :).