Considering that Blue Cyan is actually the complementary of Red, you could mix burnt umber and Prussian Blue to obtain a really fast drying underpainting for the cherries, you just need to find the correct proportion through trial and error.

This particular combination dries overnight. Now, going back to color theory, Green is the complementary of Magenta

A grisaille, by definition, is a gray-monochrome painting. Do you wish to use a gray-tinted-green color, or just mix green with white?

Personally, with the choices you’ve given, I’d mix a little sap green into neutral gray tones and use that. That’s based on the fact that A. my sap green, Michael Harding brand, dries quickly, and B. a phtalo dries slower and is way, way more powerful in chroma; you’ll be needing an atom of pthalo for a pea-sized bit of gray paint.

For an underpainting that you will be using as a base image over which to apply glaze layers of color, the most important attribute is that of value–lights, and darks. While specific colors can be used to enhance the colors that will be glazed over it, placing values in position, with their exact shapes, and positons, is more important.

As Alan recommends, a nice low-chroma, off-gray, greenish sort of an underpainting is very appropriate. It will serve the purpose effectively, and I have employed just such an underpainting in the past, for my flower, and still-life paintings.

Sometimes artists will recommend using an underpainting consisting of the complementary colors to that of the final painting, and although I have never tried that, some swear by it. If you should choose to go that route, Gigalot’s suggestion that Cyan (Thalo Blue) would be an appropriate complement to that of Red Cherries, is spot on.

You said you want to stick with what you have. Well, do it, but make sure you thin the paint with turpentine, or OMS before proceeding, this will help with the drying process.

I want to collectively thank all of you. I knew by posting I would get some great responses from some and be able to learn something. I couldn’t imagine trying to learn how to paint without this site and it never ceases to amaze me how much knowledge and willingness to help this place has. So again thank you all that have replied so far.

I second that.

If you try gray scale underpainting, then you can lost much of color saturation.
After applying transparent glaze on gray underpainting, you will get less saturated color. Complementary colored underpainting can also give you lower saturation. Then, to improve color range you must overpaint poor color with opaque chromatic paint and therefore you can miss value range. That needs a lot of routine work. When do overworking, you can lost initial idea and initial beauty with more boring painting result. However, when you do copy from digital projector using photography transfered image, the “lost of initial idea” is not a problem because all ideas are including into digital image. But if you provide your own painting idea without exact photo-transfered image, (on plein-air), then that can be a problem.

I think, that light, colorful underpainting with optimal colors can give more “fresh” result.