If it isn’t marked as a public right of way, then no, you don’t have a legally enforceable right to use them.
On an OS map, roads which are coloured show that they are surfaced – blue for motorway, red for A-road, orange/brown for B-road, yellow for unclassifed/C-road.
No colouring simply indicates that the road is unsurfaced.
A good example is Mastiles Lane in North Yorkshire. The section between Kilnsey and Mastiles Gate is only marked on OS maps as a traffic-free route (i.e. no motor traffic – widely-spaced red dots). Between these points it is not technically speaking a public right of way, but it is an unsurfaced road. Only from Mastiles Gate to Street Gate is it a public bridleway, and therefore a PRoW.