Is it possible to mix roundup concentrate (360) with water based emulsion, 1:1, before I dilute? I ask because I'm looking to kill an entire garden's worth of scrub, weeds, brambles and low quality grass (on a building site) as part of the new landscaping, and I'm finding it hard to remember/see where I've been with the knapsack sprayer. There are also a couple of trees I'd like to keep so I plan to be careful around them and wash them down with clean water after, but it would be helpful to see if there is any overspray on the leaves. Wondering if normal water based emulsion is ok for use/won't denature the roundup
Don't use paint! Any paint has binders and adhesives, so it will tend to clog your sprayer, and (as you note) you can't be sure what the interaction of the various ingredients will be with the active ingredients in your weed killer. Besides which, very little of paint is actually dye or pigment.
There are a number of dyes designed specifically for mixing with weed controls to help you see where the spray has gone. For example, Mark-It Blue.
There are a few issues with using paint:
Would take some balancing to get enough paint to be visible, but not so much that it would interfere with the operation of the sprayer. I'd suggest starting by making a dilution of paint at 1:4 with water. Obviously you need to use latex paint. Just use the paint alone, and continue to dilute until you have a just visible cover. (Of course this could be used for 'instant greening' with the choice of a suitable paint. Cheaper than green outdoor carpet...)
You need to really clean out the sprayer after. Dried paint can be difficult. Cover the nozzle if you stop for even a few minutes, as it has maximum exposure to air, and minimum hole size. An aluminum foil cap works well.
So much for the mechanical aspects.
Glyphosate deactivates on contact with clay. Paint pigment is fine particles. Depending on the particular pigment used, they may function as a de-acativator.
Paint has a bunch of ingredients in it that may react with the ingredients in the weed killer.
Baler twine is another method for tracking where you've been. Put a line along the property every 10 feet. Maintaining a consistent distance for 1 or 2 passes on either side of a line is fairly easy to do. Baling twine is cheap, and is available at any farm store.