There is, indeed, a use trail up to the tarn at the headwall of the col. How much scrambling depends on the condition of the snow field from the tarn up to the col itself. Early to mid season it will be step kicking if you wait for the sun to soften the snow. Late season it can be
very hard snow or ice, in which case bring an ice ax and possibly crampons or be prepared for a class two scramble along the left(south) margin of the snow/ice field. The trail down into Darwin Canyon is moderately steep at the top, then eases off, but you have to pay attention to pick it up at times down low until you get to the first lake. There is a stretch of talus/boulders for ~ the first 2 hundred yards along the lake, then you will pick up a use trail the rest of the way down to Darwin Bench. From there the trail can be faint at times down to the JMT. Beautiful country. BTW, there is a nice campsite on a sandy bench at the end of the above mentioned talus/boulders section, maybe 15 feet or so above the emerging use trail. Big enough for 2 tents. Lots more as the canyon opens up a mile or so later. A word of caution: You will be going from 4,000' up to 12,900' if you start from Bishop. A friend and I did that 2 years ago and got away with it, but were feeling a little woozy by the time we got down to the campsite I mentioned above. Another friend and I tried it last year and he got pretty sick and we had to bail. A better option, IMO/IME is to stop for the night at Upper Lamarck Lake and acclimatize, even though it's only ~2.5 miles from the TH. Pretty place to hang out for a few hours anyway. I'm going in that way in late July to check out McGee Lakes and possibly the Ionian Basin. It's prime backpacking country. I hope this helps, and best of luck.