Warm and fast
What do you do to add warmth to fast passages?
Bow changes need to be really smooth so that more of the duty cycle of each note can be tone production. I would play lower on the bow and make sure your wrist and fingers are nice and flexible. Those are my "amateur thoughts."
More bow? Generated by give in fingers/hand/wrist?
I would suggest more but lighter bow and moving the contact point a little bit to the fingerboard. Adding vibrato can actually sometimes help warming up the sound in fast passages. Not really vibrate, but have that vibrato feeling in the left hand. Soft fingers in the right hand can also be helpful.
Replace straight strokes with rounded ones?
Great stuff folks. No particular passage in mind.
Example exercise: in second position, on the A-string, play fast detache C-D-E-F-E. Keep a close contact with the string, play forte, good sound, middle of string, middle soundpoint or even a bit closer to bridge. Try to produce the five notes with the same tone color which is feasible as they are all on the same string. Do this many times so you get the sound and tone color in your head. Now play the same passage in first position with an open string E, so fingering 2-3-0-1-0. The sharp sound of the open E and of F are a challenge. However take it as a challenge and try to produce as much as possible the same sound as you had when playing it on the A-string. Of course there are no miracles but quite a lot is possible. This comes from Simon Fischer's Basics where he proposes this exercise of playing passages on one string and trying to reproduce as well as possible on another string.
Make sure the string rules are still being observed - ie. lower strings get less bow, more snug (or pressure), high strings get a little bit more bow and lighter. Observe lane rules (further away from the bridge in lower register) Also make sure right elbow travels with the string crossing and does not get stuck on one level. Practice passage in open strings and make it sound awesome, than add fingers. Make sure the right wrist does not become stiff, it should feel like smearing peanut butter on bread...finger flexibility. I find generally I always have to use less bow than my first attempt in the lower strings particularly.Left hand finger angle can also play a big part (experimentation is key). Good luck!