Of course we would love to be able to stock all mk whistles in a wide array of colours.  To be able to make mk whistles economically, we must anodise 10 whistles at a time of the same key.  This means that making a one-off becomes considerably more expensive (typically +50% the base cost of the whistle).   Making 10 of a more specialist key in a specific colour becomes more diffcult to justify.
For the standard keys we aim to hold all colours, and certainly for all whistles we would like to be able to offer something besides black and silver.  If you are looking for a specific colour of a certain key, which we only currently offer black/silver, then please get in touch to register your interest.  We do also consider one-offs but they would be considerably more costly as noted above.