Equations screen: change sum component color to black

[arouinfar]

@kathy-phet pointed out that it's a bit odd that the sum components on the Equations screen are dark gray, but the other vector components have the same color as their parent.

I believe we chose dark gray to improve the contrast for the on-axis projection, but now that we've shifted the components off-axis this is unnecessary.

@pixelzoom let's change the color of the sum components to match the sum color (black).

[pixelzoom]

Done in master. @arouinfar please review.

[arouinfar]

👍