Color turns Grayscale when converted from RGB to CMYK?!

I have a complex pattern, that looks brilliant with a color layer. However, it was originally created RGB.

I converted the color to CMYK for print, and all the color disappears! Not that the layer is deleted, but the color layer appears Grayscale. The only area where color shows up are in the pieces that are at 50% transparency (see purple diamond). I have all my color areas set to CMYK including the color pallet.

The color will show if I set the layer to multiply, but it doesn't look the same way as it does when its in RGB mode.

I've never come across this before, any help would be greatly appreciated! THANKS!