Krypton has a similar but slightly lower ionization voltage to that of Argon....at similar fill pressures it will produce a pearlescent whitish type color...typically it is not used for regular sign work but at higher fill pressures can be useful for effects similar to the streamers in some plasma pieces. (This is also influenced by choice of power supply) I don't have a conventional fill chart for it but do know that when it was employed in some of the T8 fluorescent lamps they ran about half the pressure of what they did when using Argon in them...sometimes a mixture was used and the lower pressure did require some changes in electrode assembly to prevent sputtering of the cathode. One other text I consulted quoted an example that was about 75% of the Argon fill pressure. You may want to create some basic test units, all of the same shape, length, and diameter...then do a variety of fill pressures to see what gives you the effect you want to see....take good notes. You can also borrow a page from plasma filling practice and connect your intended power supply while the unit is still on the pumps and slowly fill until you see the effects you want and then stop there, making note of the pressures used. Keep in mind that differing diameters may produce a different effect at the same pressure.