i do love the whole Stamen collection of tiles. they are quite beautiful and flexible. in some cases, i think the Watercolor tiles can be a little too vibrant. so if I’m using them in ArcGIS Online, sometimes i’ll make them about 70% opaque and then overlay on top World Imagery to darken it a bit and provide ghosts of features like baseball fields and buildings. when i’m working in Leaflet, i like to apply CSS style filters to the Stamen Toner and Watercolor tiles to get all sorts of interesting color combinations. For example, the first few screenshots in the “What are map tiles?” section in the tutorial are Toner labels on top of Watercolor, toned down just a bit by applying an 85% grayscale filter, and i like the resulting coppery land and steely blue water. The fourth screenshot there is Toner Lite with a series of filters applied to turn the white into an olive.
it’s also worth noting Stamen’s inspiration for the Watercolor tiles: Bicycle Portraits - a photographic book / Part III (final) by Stan Engelbrecht / Nic Grobler — Kickstarter.