Greg, you’ve completely lost me. You cannot simply replace the black toner for white toner and get proper color reproduction. That’s not how color printing works. You need cyan, magenta, yellow, AND black to get proper color reproduction. Cyan, Magenta, and Yellow, even when combined at 100% saturation, do not give you 100% black. It’s a dark grey, usually between 80% - 90% depending on the inks being used. The black is needed for that last little “pop” of contrast, as well as truer dark blues, greens, browns, etc.
Consider this illustration:

The color logo in the middle is the desired artwork. It’s fairly simple… red, blue, yellow, black, and white. To print that on white paper, the printer breaks each color into its four distinct components (cyan, magenta, yellow, and black) represented in greyscale respectively from left to right above the color artwork. That greyscale represents the amount of each color of ink the printer lays down to reproduce the color logo. The darker the shade of grey, the more ink gets laid down. For example, the red banner is made up of almost equal amounts of magenta and yellow inks, with just a trace of black and virtually no cyan. The flag banner is almost entirely yellow, with just a hint of magenta. The blue middle is predominantly cyan with a fair amount of magenta, and just a touch of black. It doesn’t print anything where there’s white, because it presumes a white background.
If you replace the black ink for white, then the printer lays down white ink in accordance with that specific image that it uses for black. As a result, you’d get lighter blue in the middle, pink around the edge, and white lettering over the yellow banner. Clearly that’s not what you’d be after.
Rather, the white layer when printing decals on clear media needs to look like the bottom–a solid block that encompasses the whole of the artwork. That goes down first, and on top of that, the CMYK layers are then printed, resulting in a properly-reproduced color logo on an opaque white background which can then be applied over any color you might choose.
The laser system you referenced in your link uses one printer dedicated to printing white toner for that foundation layer. To it, you would feed the bottom image artwork. (In Photoshop, it would be its own separate layer. Turn on that layer only, then print your file.) Step two would be to take your decal media with this solid white image printed on it, and place it in the tray of printer #2, which is dedicated CMYK. In Photoshop, turn off the layer of solid (white), and turn on the color layer of the logo. Then print to that 2nd printer.
Because it’s a completely separate printer than the first, there’s no guarantee that the paper feed will be exactly the same as it was on the first printer. In the case of this logo, there’s a thin white margin around the whole of the logo. If the paper is not properly aligned when feeding through the 2nd printer, the color component of the image (CMYK) will not print in the middle of the white foundation, resulting in uneven margins. One side will be wider than the other. It’s possible that it may print right to the edge or even off the edge of the white foundation. That’s dependent on the printer and the mechanics of the paper feed. If the artwork did not have a solid white border around it, any misalignment would show up as a white “halo” along the edge of the artwork.
That’s the specific advantage of the ALPS printer. Since the paper never leaves the paper feed, much less the printer, the image will remain in perfect alignment for each subsequent pass of each color. When you’re printing decals for model railroad equipment (even in our large scales), you’re often dealing with fonts as small as 6 points, if not even smaller. There’s not a whole lot of room for misalignment there, which is why the laser printer process has so far been passed over in favor of the more accurate ALPS process.
Now, there are tricks you can do with the artwork so that there’s a bit of a bleed with regard to the color over the white foundation to avoid the halo effect. You’ve just doubled (or more) the amount of time it will take you to draw your artwork because your background white layer is no longer a simple matte cut-out of the artwork, rather it has to be adjusted proportionally so the colors bleed consistently over all the edges. It’s not just a matter of scaling it; you’ve got to adjust the thickness across everything so that open areas inside the artwork still properly bleed over the edges. It’s not a fun process, and there’s no “Photoshop shortcut” to make it easy. (I do this a lot with matte keys for video–same process.)
In terms of your assessment of what you call “the real issue” of getting other colors in the white, that’s why the OKI printers you referenced are sold as a pair, with one printer specifically dedicated ONLY to white toner, and the other for the CMYK print process.
Later,
K