Watercolour wash and line was the popular addition to paintings from the late 19th Century onwards. Then double colour mounts reduced its popularity; plus there was less work and skill involved. Wash and line was deemed to raise an art work out of the Hobbyist category and place it among professionals.
We do wash and line. You can choose colours, how any lines, widths. It’s applied by hand so it’s versatile. Used with discretion and not as an overstatement it will add interest and focus to all sorts of works. Even one black line around a photgraph has considerable impact for the subject. Prices depend on the size of mount and number of lines but rarely exceed £10-£12 on a large work and can be as low as £2.50 on something small. Below is a recent example to illustrate the effects.