"My ballast is the right

How about airbrushing a lighter color over it? ......DaveB 

Darkening

We put down Arizona Rock & Mineral ballast using thinned white glue, with an alcohol pre-spray to help the glue mixture penetrate. Immediately after the glue set up, it was definitely a couple of shades darker, but pictures taken over several months show it has again lightened up to very close to the original unglued color.

Same ballast?

Ballast might be the difference if you're using something permeable, like Woodland Scenics.

The ballast is scenic express light cinders

the ballast that darkened is scenic express.  I haven't laid it yet.  I did a test area.

Sam

Rock Ballast

Actual rock ballast is less prone to darkening.  Look for brands such as Scenic Express and others who use actual crushed rock.

Stay away from Woodland Scenics if your worried about color change.

Joe

I just used dilute craft paint.

The original woodland scenic ballast I put down 15 years ago was too dark.   I just mixed up a wash of cheap craft paint with water and quickly went over the ballast.  It also weathers the ties lighter(too light, and will need a little toning down later, but that was just cause I did it so fast and sloppy between the rails)   For a slightly distant shot, go down to the 'franken-yard' entry on my blog, and you can see the change in color from the original on the left 3 tracks to the after on the middle-right 3 tracks.

http://model-railroad-hobbyist.com/node/17314?page=1

obviously, practice on an out of the way place first.   I used just straight white since that was near where I wanted to end up.  You may need to find a different color based on what you are starting with and where you want it to end up.   It only took a couple hours to do seven yard tracks that are about 15 feet long each.

Inconsistency of product

After reading a lot about HO scale ballast, I went with AZ Rock. I used "Yard Mix" and it darkened to nearly black. 2 yrs. later, still very dark. I called them and they said to expect that darkening. I told them what I was going for and they suggested #1383. They were right, it darkened to exactly what I needed, however 1382 would have been a better choice size wise. The last 2 bags I purchased not only were VERY dusty, I had to sift it twice, it did NOT darken at all so there is a visible transition to a lighter mix.

In hind sight, I think I would have been better off with Scenic Express's SE0353 #40, blended gray for size AND color. Too late now as the layout is well over 1/2 ballasted.

Next time.

Real ballast is not uniform

Keep in mind on railroads ballast varies in color etc.

I second using real rock ballast.  Not Woodland Scenic.

Maybe

Might be able to blend the old dark stuff with the new light stuff by sprinkling a light application of the light dust sifted out of the most recent delivery.