In his Old Ireland in Colour project at the Insight SFI Research Centre at NUI Galway, Professor John Breslin uses DeOldify, an open source tool, to bring colour to archive images. This photo from the National Library of Ireland's collection shows a Republican prayer vigil at Downing Street, London, July 14th 1921, as Éamon de Valera and David Lloyd George met to discuss paths to peace in Ireland. 

This particular image posed a challenge for Breslin. "I hate crowds," he says."Not because I'm anti-social (I am), but because you have to work extra hard to colourise them!" So how did he choose what colours to use. "I used two different commercial implementations of DeOldify (MyHeritage.com, ColorizeImages.com) to give a mixture of clothes colours, and adjusted the results to have four main shades overall: two kinds of brown, navy/grey and some green. I added other hats/clothes colours manually as well."

Crowds can be particularly tricky because it's easy to miss things. "When there are crowds, you have to take extra care to make sure you haven't missed out on any faces (partial) or hands, but also to ensure there is some consistency and variability between skin tones," says Breslin. "I changed various eye colours to blue to match Irish norms. Irish flags of course, and there's a small sacred heart image on the flag which was given reddish/brown tones. 

I also fixed various marks/scratches on the image. You'll see various women wearing gloves in the original, on both hands or just one hand. I believe the colour makes some elements of this photo stand out even more - the girl in white at the back, the people holding the flags, the shillelagh or walking stick held by the woman in the front who is also wearing a harp insignia."

And sometimes, the people in the photo have got things wrong! "I checked with Matt Loughrey of My Colorful Past in Mayo," says Breslin. "He also spot-checked the colours in the sepia version and confirmed the hunch that the flag at the back is being held the wrong way around!

Original image courtesy of the National Library of Ireland