I would never use 6500k cobs, even in a veg only tent. It's just way too much blue and you might find that your plants are too compact and might even see morphogenic issues. 3000k is great for veg, though my blurples grew more compact than 3000k for sure. 3500k is perfect for veg, and maybe even 4000k if you really like compact plants. I feel a 50/50 mix of 3000k and 3500k (what i use in my tent) makes the plants grow the perfect strechiness. Compact enough that you get lots of tops, dense growth, thick stems, and good leaf size but stretchy enough that lollipopping/trimming isnt an insane undertaking or to block airflow in the canopy and to make it easy to see issues.
Blue is also more stressful to the plant because it's high energy (almost UV).Having the crazy amount of blue that 6500k cobs output is going to make photosynthesis less efficient than 3500k by causing some stress and the lack of red (main photosynthetic driver).
IIRC I believe a 3000k or 3500k cob has more blue than a typical MH bulb outputs.