The problem you had with white paper may not be with the paper being white, but with the rasterizer in Flux Studio that processes your image into gcode. I tried engraving the “Mayan astronaut” image (the engraving on the tomb lid for Kin’ich Janaab Pacal) I downloaded from Wikipedia as an SVG image onto a comic book backing board (they’re a convenient size, white on both sides, one side gloss, other side matte), and it came out well, except that there were extraneous straight lines in the engraving that weren’t in the image when I viewed it in Irfanview. I converted the image to a PNG bitmap with an online tool (nice fine, sharp lines, very clean) and tried engraving the bitmap, and all I got was white – the laser head never drew anything.
I suspect that what is causing this to happen is that the resolution of the laser when engraving from a large bitmap is lower than the resolution of the image, so the rasterizer processes 2x2, 3x3, 4x4, etc. blocks of pixels from the original down to single pixels for the laser to burn (going from black-and-white to greyscale in the process), and the pixels in the shrunk image fall below the threshold for the Flux to draw that pixel as black.
So if you had a threshold of 50, and your image consisted of one-pixel-wide vertical lines spaced five pixels apart, and the rasterizer converted the image by converting 3x3 blocks of pixels into individual pixels for the laser to draw, each 9-cell block would have 6 white pixels and 3 white pixels In a vertical line, so that block would be 3/9 black, or 33%, which is below the 50 threshold, so the entire image would draw as white.