What about Fund raising?
Even the appropriate paints cost money! Local funding is best, because local people involved in a local project will help ensure the future of the restored milestone.
Approach the Parish Council. Seek sponsorship from a local business. Look for a local charity, including local family charitable trusts.
If approaching a national funding source, like the Heritage Lottery, bigger projects may be easier to fund than smaller ones. It can be easier to raise funds to restore the milestones along a whole length of road, than just one individual milestone, though matched effort on public engagement is usually required.
Consider adding a plaque to the restored milestone or located nearby to tell people about the restoration, with a date and possibly the names of sponsors.
Good publicity is often a key to fundraising – you may find sympathetic local press or social media contacts.