Try pressure washing them first to see it you like them cleaned up. If you do want to change the look you can acid etch them or stain them- there are lots of how to videos online but basically you need to clean the slabs with a degreaser, apply your stain with a roller (or acid with a spray and 8-24 hours later a neutralizer), and then seal the finished product. There are options at the DIY store or online- personally I would be looking to buy a combined stain and sealer designed for concrete slabs.
Looked into this a few weeks ago and came across this: https://www.adsealcolours.co.uk/Smart-Colour-Stain.html As Jonathan said, power wash the slabs first as you may like them but degrease, rinse and let dry before adding any colours.
Jonathan
rachelmidlands
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