I use, and have for years used, the API Freshwater Master kit. You can get it on Amazon relatively cheap. It's reasonably accurate.
pH pens are also getting cheap, and if you're testing ph frequently, is a bit more accurate than trying to compare color charts.
ANY time you compare color charts, there's a degree of inaccuracy. How much accuracy is needed depends upon what you're testing! For example, when testing an aquarium during the cycling process (when dosing ammonia), 0ppm ammonia and 0ppm nitrite are what you're shooting for. In an unplanted tank Nitrates will be through the roof, but you'll simply change the water before stocking. 0 is 0, and is fairly clear on the tests.
When testing nitrates to determine dosing amounts, there's a degree of inaccuracy looking at the colors, and, as mentioned; they need to be calibrated.