All the pretty colors. Great when in a rainbow. Bad when in your spa. Spa water should be crystal clear. So when your spa water is yellow, what does that mean?
Here’s a quick synopsis of what these colors mean:
Cloudy Green
Dude, you got algae in the water.
Treatment: Shock your water with chlorine.
See-Through Green or Emerald Green
Acidic Water. The metals in your heating elements are being stripped. The average cost for a replacement heating element–$200-$250 big green dollars (includes labor).
Treatment: Try a product like Metal Gon by Leisure Time–then balance your chemistry.
Total Alalinity 80-120, pH 7.2-7.6, . If this fails, drain and start all over.
Yellowish
Indicates low total alkalinity and low pH. It’s also the precursor to damaging the spa’s heating element.
Treatment: Balance your total alkalinity and pH. Also check your calcium hardness — should be over 250 ppm.
Treat with calcium increaser.
Blue
Blue rarely happens in above ground spas. But in-ground gunite spas– with gas-fired heaters–can experience bluish/turquoise water, as well as the same color stains on the plaster.
Treatment: Get the total alkalinity pH, and calcium hardness balanced. Also check total dissolved solids (TDS) –not over 1500 ppm– because that can also begin striping your heater.
Brown
Rare, but happens. Usually indicates iron in the water.
Treatment: Metal Gon should cure it. If it continues, drain, refill and add a bottle of Metal Gon when you refill.
Pool & Spa Master 
My hot tub is new (three weeks old) I’m confused about how much chemiclas to use and when???
Thanks for the question. The place where you bought your spa should be eager to help you. However, that’s not always how it works. Since your spa is new, I’m sure that you, family and friends are using the beejezus out of it. So, it probably requires more chems right now than it will this time next year. Here are the rules of thumb:
1) Balance total alkalinity
2) Balance pH afterwards
3) Add shock
4) Add sanitizer
These are the basics. I’m happy to talk with you personally for free advice. Let me know if that is what you require for healthy water and relaxing spaing.