Playing with pears and grisaille painting - yep, you can do it in watercolor but I'm not sure why you would want to do it in watercolor. This was a lesson shared on WetCanvas (http://www.wetcanvas.com) from a member who tried it and got a much better outcome than I - you might do a search to try to find it there, if you can (I'm not sure how long I've had this lesson lying around as a printout I intended to try sometime). The member's name is Asel Syzdykova and she goes by the member ID Aselka at WetCanvas.
In the demo, she used a mix of Prussian Blue and Burnt Umber (she doesn't say what brand) to create a grey underpainting, putting in the shadow shapes of the pear before putting in any color.
My mix of PB and BU was a bit too greenish or too brownish and I never got what I would call a good neutral grey using Daniel Smith colors. But I tried it anyway.
My front pear is done this way.
The back pear is done using transparent colors of Hansa Yellow Light, Hansa Yellow Medium, Quinacridone Gold, Quinacridone Coral and a touch of Perylene Green.
Not a good painting but you get the idea - and the fact that, when doing an underpainting or grisalle in watercolor, you do lose what vibrant light watercolor has - if I went in again on the darker pear, I could get it more modelled and more like an oil painted pear but I think trying this has proven it's not something I want to do again. I like the light of watercolor too much.
Have you ever underpainted a watercolor painting? What colors did you use? Did it work out for you or did you think it muddied things up too much? (Now maybe you have a lighter hand than I and can do underpaintings well - if so, you might want to give this a try.)