Hi Barb,
Bright yellow? Are you referring to an OTO test? If so, you don't have to check with 2 tests at all there's no reason to do that. Since you've got the TF-100 just use that when you need to check, and I would suggest checking it tonight, and in the morning. Also be sure you are using the 10ml mark on any tests you can, instead of the 25ml because it uses less reagents too. You can also get away with 1 scoop of powder instead of 2 as long as it turns pink, it will be accurate, and that will save reagents too. (of course any question on it recheck with 2 scoops, but I used 1 a lot, and all mine were fine).
Being 18ppm actually it means you need to Maintain that, and not drop below 16ppm for the night.
When you have algae it can eat through the chlorine fairly rapidly sometimes, especially at the beginning of your SLAM. So with your Slam just starting out you don't want to drop below 16ppm. Tonight with there being no sun you'll lose less, than you would if it was daytime, but will still lose some. Possible 2-4ppm drop overnight depending on what's in there.
Thankfully, the chlorine loss will get less, and less, as more, and more algae is killed off, so you'll need to compensate less as time goes on.
If it were me, lol because I'm a worry wart, and like to be better safe than sorry. If I decided not to check it tonight, I would at least add about a 1/2 gallon, or so, of chlorine before turning in tonight just to be on the safe side, at least, until you can determine approx. how much you will be losing for sure. A half gallon isn't going to hurt it a bit either way, and may be the difference in being at 16ppm in the morning, and 15.5ppm in the morning.
Hopefully it will hold, and it will be 18ppm in the morning but with it being green it's going to take a few night I think to get the levels holding well. If it does hold, that's great, and that's just another 1/2 gallon of cleaning accomplished overnight LOL whoo hoo.
. But anyway, the decision is yours on that.
I am disabled, and was only able to physically do checks/add chlorine every 3-4 hours, so I had to run my levels slightly higher than most people, but at least I never had to worry about falling below SLAM/Shock level, and my pool is now crystal clear.
Have a wonderful night.