Scott's/ RoundUp Concentrate directions warn against mixing in marker dye before applying to lawn & weeds.

[MowerMark]

Were you aware of this? The dye supposedly decreases the potency of the glyphosate.

Scott's recommends 2.6 oz. RU concentrate per gallon of water since they classify Nimblewill as a difficult weed to kill;.

 

[bertsmobile1]

Very few does will chemically affect glyco unless they are glycol based
SO they are the common powder dies plumbers use to trace leaks, green, yellow, red or purple .
Horticultural dyes should not effect any spray as they are chemically stable but UV unstable

 

[MParr]

Read the label of the particular dye marking agent.
The Roundup label states that a dye or marking agent “can” reduce the effectiveness. That doesn’t mean it “will” reduce the effectiveness.
I would go ahead with the application and monitor the effectiveness. Give it 7 days to work. If you don’t get good results, hit it again.
The Nimblewill sounds as though it’s as hard to control as Common Bermuda.