It's a hawk-moth caterpillar (family Sphingidae). I think it's the Broad-bordered Bee Hawk-mothHemaris fuciformis, in that stage when it searches a place to pupate, so it changes colour. Check the images on this page (click to enlarge, then scroll down). You can see green ones and the brown one.
That will be the Lime Hawkmoth, Mimas tiliae, displaying pre-pupatory colour form. This provides better protection from predators than the vivid green that they are earlier as they wander in search of a good spot in the soil to pupate.