What is the colour of ${\mathbf{K}}^{+}$ through cobalt/ double blue glass?

The colour of 'K" through cobalt/ double blue glass is Crimson red.

 On the colour wheel, crimson is a reddish magenta shade that lies between red and rose. It may be created by mixing a little blue with a powerful, vibrant deep red.

 Glass made from cobalt is often a rich, deep blue, like the vase on the left. Cobalt oxide is added to the molten glass mixture to create it. The majority of blue glass has either copper oxide or cobalt oxide added to the molten glass to give it its colour.

 Hence, option D is correct.