If it's a street tree it very well could be. Could possibly just be fresh growth and no sun.

Seasonal magnesium deficiency.

Could it be this http://www.songsparrow.com/catalog/plantdetails.cfm?ID=4945&type=WOODY,&pagetype=plantdetails&plant_name=Syringa%20reticulata%20%27China%20Gold%27 ?

I remember that these trees were yellow-ish in summer time when I saw them last year. I don't recall lilac flowers but I'm not often in the area so I could have just missed them. I'll be on the lookout for flowers this year!

They do get plenty of sun - I would say full exposure.

This species often leafs out yellow or pale green here, if not everywhere - plant asked about does not have to be a colored foliage selection.

A similar species, also in general commerce is S. pekinensis. It will become much easier to tell later in the season.

I have China Gold and Summer Snow. The former emerges gold/yellow maturing to dark green. The latter emerges pale green maturing to dark green.