Hope I can help someone with my hard earned lessons in Koi pond building. There is a lot of marketing of many types of concrete sealers, and I have tried a lot of them. Look at the warranty. They all say how great their sealing is, but will they stand behind it? Can you talk to pond owner who used their product 10 years ago? Believe it or not, the cost is only slightly related to effectiveness. My favorites in order of best to worst are: Crystalline (Reactive Silica), Pebble tech type Plaster, Rubbers (Polyurea bedliner, Urethanes, EPDM, etc), Epoxy (which is a waist of money). You may also want consider integral sealer which is added to into the concrete mix prior to applying (shotcrete, gunnite, or pored if its a gentle slope). The best bond for cementious based coatings is a rough open pore green concrete (free of efflorescence). As the concrete ages the bonding method changes from a chemical bond to a purely mechanical filler bond needing even better prep. The rubber and epoxy based concretes usually needs a dry open poor rough surface. Prepping the surface is very critical for good bonding to the concrete. Make sure to follow the recommended directions. Always error on more prep. What I don't like about the rubber is the cost, and the epoxy doesn't last, it flakes off after 5 to 10 years.
Now to talk about my favorite coating by far, the Crystalline (Fumed Silica based type). I use Zypex (there are other brands which I have not tried yet: Kryton,Permaquick, Penetron, Vandex, etc. Just Google crystalline water proofing). It is a cementious based coating which soaks into the concrete and reacts with water in the to create a non-water soluble silica base crystals (jell) deep inside the concrete. It fills in the open capilaries in the concrete. After proper application, it will sit dormant in the concrete until moisture is present which makes it grow the waterproof crystals. It grows towards where the water is, filling cracks and pores. Its self sealing and will self repair future cracks up to .4mm (which is a pretty big crack for reinforced concrete). It is non-toxic, used for dams, concrete potable water tanks, fish ponds, underground parking structures, etc.
I had 8 foot deep 20,000 gallon pebble tech coated cement pond that sat empty for an entire year after a large leak formed dropping the water level 6 inches a day. It was acid etched to open the pores and one coat of Zypex was applied (three 5 gallon tubes of Xypex concentrate at $160 each) and that did it, no more leaks! The great thing about this coating is that its permanent, it will remain dormant until moisture is present which will make waterproof crystals grow into the cracks and seal them. Wish I have found this stuff 20 years ago. The pond is full and future cracks will seal as they form.
Side note: Color only matters for the portion above the waterline. Within a short time it will be covered with Algae, so simple gray is fine by me. In California we get a black algae so why add color.