Ah, found it - a photo dated 1910
A gentleman on the left appears to have naturally dark skin.
This picture was used in Doc Rowe's book "We'll call once more unto your house", printed in 1982.
Underneath the picture it says "There were lots of fishing boats here at that time from the East coast.....Yarmouth and Lowestoft. Mayday was nearly at the end of their season, from January until mid-April, and they used to stop over - 'cause it's a tidal harbour they could scrape and paint the bottoms of the boats....."
"Not being Padstow born.... they loved a good booze-up and it became rather disreputable.....
These people became more native than the 'natives' on Mayday! One lot even pinched a 'Obby 'Oss and took it back with them."
Oh, they had some characters...that's how Captain Dolly became part of the 'Oss party - he got dressed up as a woman - he was a Lowestoft skipper." (Captain Henry Dennant)
There's another photo from 1910 with a banjo
but no coloured faces. There's also a photo from 1913 in Doc's book (but I can't find it online) showing another banjo, held by 'Captain Dolly' who is wearing a long stripey coat on this occasion.
So maybe the painted faces date from just before WW1 but I don't think it's much earlier.
Robyn