Modern European and Western harps almost always use a colour coding system to help the player identify the pitches of different strings. The F strings are coloured blue or black, and the C strings are coloured red. You can see this clearly indicated on a print in Charles Egan's Harp Primer of 1829.
Harps originating from outside of Europe however do not use such a system. Koras, Burmese harps, African harps etc. have all of the strings uncoloured and unmarked; the musician has to know his way around the instrument.
Present day historical harp players almost all use the modern European colour system on replica instruments or speculative reconstructions.
What is the evidence for the historical origins of the three-colour system (red, black, uncoloured) that is now universal in Europe? Can we come to any conclusions as to whether medieval, renaissance or baroque European or Gaelic harps had coloured or marked strings? This page gathers evidence.
19th century references
By the early 19th century the 3 colour system (C red, F blue or black, the rest uncoloured) was standard on pedal harps:
The pedal harp system seems to have spread quickly to other European harp traditions; in the mid 19th century the Welsh triple-harp player Ellis Roberts (1819-1873) described the 3 colour system in use on welsh triple harps. He describes how to locate the F (or F#, the text is unclear) strings:
18th century references
17th century references
16th century references
A painting in Pollock House, Glasgow, shows a single-row Spanish harp with some coloured strings3. I have not yet seen a good reproduction of this painting.
Fray Juan Bermudo in his book 'Declaración' of 1555 discusses the limitations of Spanish diatonic harps, and ways in which chromatic notes can be played on them. One suggestion is that eight new strings would be added to the row of natural notes; to distinguish these chromatic notes (2 or 3 per octave) from the naturals, the chromatic strings would be coloured red, to make them recognisable against the naturally coloured diatonic notes.4 He does not say how the colour might be produced but plain gut strings as used on 16th century Spanish harps would be easily dyed with vegetable based dyes.
This is a very different system to that used on modern European harps and indeed on modern replicas of 16th and 17th century Spanish harps - Bermudos implies that the A,B,C,D,E,F,and G strings are uncoloured while Bb and Eb are red.
The strong implication of this suggestion is that 16th century Spanish diatonic harps had uncoloured gut strings.
In 1581, Vincenzio Galilei mentions coloured gut strings in his Dialogue. It is not clear what the colour is for though.
Early Irish harps have metal strings. I do not know of any historical method to apply colour to monofilament wire strings without affecting their strength and/or density (either of which would spoil them as music strings). Present day early Irish harp players who colour their strings usually use xylene based permanent marker pens, or more resilient water-based synthetic marker pens such as the ones I carry in the Emporium. Needless to say these are not at all historically plausible.