This green caterpillar has kept me busy for a while. Could be an owlet moth, but if you are thinking of the Distinct Quaker, where are it's white stripes? Did you see it walk? Did it "inch" its way forward like a "looper"? (Geometridae) Which plant did you spot it on? Surely not the Santoline? (Does anyone eat santoline?) If on a cabbage it might be a cabbage white.... A curious mystery still. Depends also where you live - clearly not on a latitude north of Paris where everything is still way too cold for your lovely selection of stirring spring!
Happy gardening!