Donald, I use the blue dye in my tank sprayer when I apply weed killer and it is definitely a valuable asset. If you are spot-treating weeds it makes it much easier to see what you've sprayed and what you haven't. However,,, it's a mess!! One tiny drop gets on everything. I have blue stains in my driveway that seem to last for months from where I put a squirt of the dye in my 2.5 gallon handheld tank sprayer.
I bought a quart of the stuff a couple of years back and I mix it with some cheap dish detergent that I use as a surfactant to help keep the weed killer on the leaves of the target weed. So now I have the blue dye mixed in a 15 oz. squirt bottle. It has helped with keeping the dye from splashing and dripping all over the place but I strongly recommend you wear latex gloves when working with it.
So, get some and use it. It's water soluble but it will stain for a short period of time.