Good grief - no one's ever suggested clothespins!  That's a GREAT idea!
There is no white added to the picture in watercolor or any other medium -  the white that's there is just white space that was left unpainted.  I worked the sky wet on wet but didn't do a wash over the whole sky but instead left areas untouched by the water to preserve the white in the clouds.
Do you recommend using white?  The only white I've found in watercolors here is Chinese White and I find it to be sort of greenish, plus it makes the colors opaque or muddy rather than transparent.

This is a page from my Field Book with a quick watercolor study that I did on location back in the Fall.  The location is Pemigewasset Overlook at the top of the Kancamagus Pass in the White Mountains of NH, elevation somewhere above 3000 ft.  Even though it was early October the temp was below 30F and WINDY.  Total painting time was only about 20 minutes before we had to run to the car for shelter and get the heater on!
I like the FEEL of this study well enough that I've considered framing it even in its present state.  I ultimately worked up a small (8x10) acrylic from it (which I won't post because I also used my reference photos to complete it) but I'm wondering if someone could offer advice/pointers on how I might be more efficient and have a quick study like this be closer to a viable finished piece?
No comments on the blue painters tape, please - I know white is more preferable but it was unavailable (awaiting shipment) at any of my local stores and it was either buy the blue or not go out painting!
Thanks!
Sharon