We have a number of these caterpillars in the yard. They are about 3 cm long. This is on the peninsula south of San Francisco, CA, USA. I initially thought of geometer moths but it seems to have the full complement of prolegs.
This sure looks like your little guy!
Looks like it may be an American Angle Shades moth caterpillar:
The cabbage looper, Trichoplusia ni, is a moth, and has 3 pairs of prolegs.
The imported cabbage worm, Pieris rapae, is a butterfly, and has 5 pairs. The intensity of the coloration vaires with region and diet. I also see in other examples that the hindmost proleg is often smaller and almost vestigial, and might just be hard to see from the angle of your image. If you view another, look for that 5th leg!
But no matter which, they are a terrible nuisance to any cruciferous garden crop.
(I used mainly U of Florida for anatomical and taxonomic confirmation, but their photos aren't the best.)