Hi Tom, just found that neither program will work with black and white images only colour, so back to the drawing board.
Many 3rd party plugins, as well as many native Photoshop filters don't work on grayscale images, just like a few years ago, many plugins and native PS filters didn't work on 16 bit images or under a 64 bit OS.
The incompatibility of the NIK plugins with grayscale images is no big deal whatsoever because you can simply change the mode of the image to RGB, run the NIK plugins, do any other manipulations in RGB, and then, if for some bizarre reason you really want to go back to grayscale, just go back to Image -> Mode, and just select "Grayscale" when you have completed your work on the image.
However, to be honest, in the hundreds of thousands of images I have dealt with, I have hardly ever used the grayscale mode because there are almost no benefits to doing this, but there are disadvantages.
The main "advantage" is that the file sizes are smaller, but, with the current extraordinarily low prices for storage and data bandwidth, unless you are storing and moving huge images (ie, tens of thousands of pixels on a side), this is almost a moot point. IMHO, there is really only one valid reason to use grayscale mode (at 8 bits per channel), and that is that it gives you access to other relatively obscure Photoshop processing modes such as bitmap, halftone screens, duotones, tritones, etc.
The disadvantages to working in, and distributing grayscale images include the one you just discovered, and in addition, because 99.99% of all digital images (say, on the web) are RGB, there is viewing software out there that just doesn't understand grayscale, CMYK, or other variants of RGB such as ProFoto RGB, so you are just making a potential headache for yourself by distributing images in grayscale instead of in the form of a desaturated sRGB image. If your grayscale images are seen by a wide variety of people, eventually you will start receiving a low but steady stream of complaints from viewers using either ancient or cheap software that can't display them.
Bottom line: Convert to RGB and you'll be fine.
Tom M