Perhaps because it may not be Riihimaki. I know some geometric shaped Aseda Glasbruk vases from the 1960's had coloured bases of at least green, blue and amber. I actually own one myself with an amber base. There are also some displayed on page 26 of that fine publication by Leslie Pina, Scandinavian Glass 1930 - 2000, Fire & Sea. If you can show a complete photo of your vase I may be able to confirm.
Hope this helps,