Apart from your jersey, once the cold sets in the best thing is to have a good jacket. Let's not forget, trial is a mountain sport. Yes, you can practise it in the most unexpected places, but out the open countryside is its natural territory.
You can always find a compromise between cooling and thermal capacity. Which is why manufactures such as Hebo work with materials that are able to provide both properties...with removable sleeves, that never fail when it comes to equilibrating our temperature and affording us greater mobility.
Also, the numerous pockets they have are very practical, with removable thermal linings that we can complement them with thermal clothes. Also, they come prepared for installing protections, such that they're practical, without sacrificing safety.
The sizes are varied, but there's even some for children, so without a doubt you'll be able to find the perfect one for you. And all the colours: from the brightest, to the most discrete.