n.the application of very thin coat of color over the surface of a picture
英汉双解大词典
scumble/ˈskʌmbəl/
1.
V（动词）
(in painting and drawing) to soften or blend (an outline or colour) with an upper coat of opaque colour, applied very thinly 薄涂暗色使(绘画中的轮廓线或颜色)变柔和
2.
N（名词）
the upper layer of colour applied in this way 薄涂的彩色