Hi
If it a matter of colours/contrast and brightness then I think the best way to match the clips is to preview them side by side.It's difficult if you are guessing by eye. (but see alternative method at bottom)
If we want to match Clip 1 with Clip 2, place Clip 1 on Video Track 1 and Clip 2 on Video Track 2 (the overlay track) but move it so it is above Clip1. Put the red cursor in the centre of the two clips. Note that Clip 2 the one you are matching to will cover up Clip1 at this point.
Now select Clip 2 and click the effects button (marked FX) Click "Add effect" and select the Crop effect. Change the x1% to 50% and then click "Apply."
On returning the the main screen you should now see both clips 1/2 and 1/2. side by side
Select FX for Clip 1 and choose the color effects. Depending on what the difference is you should now be able to match the colours; contrast etc. of Clip 1 so it matches with Clip 2. Being side by side comparing the two clips is far easier.
Obviously, if you need to match Clip 2 to Clip 1, then that's the one to go on the main video track.
Once you have matched Clip 1 as close as you can get to Clip2 , just set the Clip 2 crop x1% value back to 100, "Apply" and pull the clip back onto the main track.
An alternative (which won't always work, particularly if the clips are very dark) is to select the "Auto Levels" effect. And apply to all the clips concerned.
Nat