I'm guessing, that you are:
HEADLIGHTS and TAILLIGHTS
I am one, my twin is another
Moving along, you might see either of us
Although which one you see, depends on where you look
Depending on whether you look at the cars ahead of you, or behind you, or in the opposite lane, you'll see either headlights or taillights
Only at night are we truly very vivid
Although they can be seen in daytime as well, they truly 'light up' at night
Speed and I are related, although distantly
But not as distant as you think, as you see me often enough
You can gauge the speed of a vehicle at night based on its lights
The vehicle's speed is also determined at night by the throw of its headlights (i.e. don't drive faster than you can see)
On bicycles with dynamo-driven headlights, the brightness of the headlight is proportionate to the speed of the bicycle
Further, as pointed out by NL628 via a comment (thanks NL628!), if the car is braking, the taillight would be red, so if you brake, the red color appears.
My twin can show you the way, but I certainly cannot.
For every which way you look, I face the other way
Headlights are used to illuminate the path ahead, i.e. the path of travel
Taillights illuminate you for the cars behind (i.e. the other way)
I am a something you must name specifically,
And my twin is another you must name too
Hence my answer that you're the headlights and taillights on a vehicle.