Painted on clothes
I haven't tried it myself - well not as a pretend clothing. There are many who have - see Body paint and Balloon Creations group. I think it is easier for women as men have a certain appendage which is hard to disguise. But it has a good chance of working if the shorts are a dark colour.
Given the current state of the law together with the latest official police guidance on the subject, I would think that such body painting is totally inappropriate for a walk in the UK countryside. Total unadorned nudity would be better.
i have seen some very well done body art on both men and women where even when you know they are nude it is hard to tell because of the well done paint job. Would love to be a canvas for someone someday.
On a more practical level, i have a pair of barely there shorts which are a pair of cotton boxers with the crotch material cut away and the sides slit so it leaves me wearing an elastic waist band with flaps of fabric front and back. I try to have them sort of flesh covered. They wad up to next to nothing to have in my hand to carry with me on walk or I keep handy if doing something nude where i might get spotted. I figure I can slip those if spotted from afar and if the person gets closer they may have just thought they saw a nude guy. I have worn them while upclose with people like a couple of weeks ago with the lumberjacks who were in the woods behind the house. They made no comment about how skimpy my attire was.
A magistrate once told me (and a lot of other nudists) that a painted body is a work of art - no longer to be treated as a nude. I agree with johngw that body painting might not be appropriate for a walk in the country. As a work of art it could get by in Birmingham. Be warned though - most body paints wash off easily so don't depend on it in the rain or on a hot (sweaty) day.
I've been naked in a city center early in the morning and must have been caught on CCTV. They may have tracked me just for amusement. but they have better things to do (more serious crime) than chasing a nudist. Also the English law has no objection to pure nudism or even streaking so any prosecution would fail. Anyhow the CCTV would probably not be good enough to see the shorts are painted on.