here in Germany we have difficulties to get Gunze colors. Is Tamiya X-23 a good replacement for the Gunze Color ?
Certainly not. X-23 is clear blue whereas RLM76 is a kind of blue grey.
If you want to use Tamiya colors, you should mix basic light grey with maybe 10-15 % of blue. Otherwise try Internet (ebay, etc.) for Gunze colors.