Sanyo Valcan level line was a lo-vis fluorocarbon that was considerably stealthier than brightly colored, fluorescent, hi-vis fluorocarbon tenkara lines. For a long time, I believed that hi-vis fluorocarbon tenkara lines did not alert the fish. I did recognize that my catch rate was better if I made sure to keep the colored line off the water, but I caught fish - lots of fish - with bright, fluorescent orange line.
Recently, though, Rob Worthing has convinced me that just keeping the line off the water might not be sufficient - at least if you are fishing for highly pressured fish in relatively clear water - particularly on a bright sunny day. He tested bright lines and stealthy lines and says there are fish (very large, very wary fish) that you just cannot catch if you use a brightly colored line.
For a while now, I have offered clear fluorocarbon tippet material in diameters equivalent to Japanese tenkara line sizes as a stealthy alternative to bright, fluorescent tenkara line. If you use a clear line you definitely need to use a sighter to detect strikes, though, and the TenkaraBum Tactical Nymphing Sighter was introduced for just that purpose.
The Sanyo Valcan had been another option but it has been discontinued by Sanyo. I will look for other options, but for now I would suggest using heavy fluorocarbon tippet material. The -2X tippet is the same diameter as size 4 tenkara line. The 0X is the same as size 3 line, and the 1X is the same as size 2.5 line.