We know it can be frustrating when cards and envelope colors don't match. The reason is that envelopes are printed on three different types of devices depending on colors. These processes are Inkjet for black ink envelopes and laser for color envelopes.
Also the resolutions are different. The Indigos print at 812 dpi at 8 bit, addressability: 2438 x 2438 dpi HDI (High Definition Imaging) and the other devices are 1200 dpi.
These two factors combined with the fact that the papers are different thicknesses even if they are the same stock make it impossible to match colors. We do try and some colors are close (obviously black is the closest).
We recommend not trying to match the colors exactly and if complimentary colors work that is certainly encouraged.
We hope this helps you design to satisfy your customers better.