If you have another 18.93l pot, fill it with the same media as your plant and put it on some bathroom scales- that weight is your dry pot weight. You can either keep weighing plants when you think their near their dry pot weight to know when to water, or do it by feel (my feel is rubbish, so I used scales). When you water, say 3l, that's 3kg. Weigh them for a few days after watering to see how much they drink. If that's 0.5l/day and you have given them 3.0l, you know that's ~6 days of water. etc etc.
Regarding the lights, veg lights will be blue & white, red is more for flowering. If you have plants that normally grow short, you can use the red as well to create some extra stretch. HTH and Happy Growing!