Chlorosis is typically caused when leaves do not have enough nutrients to synthesize all the chlorophyll they need. It can be brought about by a combination of factors including:

a specific mineral deficiency in the soil, such as iron or magnesium
deficient nitrogen and/or proteins
a soil pH at which minerals become unavailable for absorption by the roots
poor drainage (waterlogged roots)
damaged and/or compacted roots
pesticides and particularly herbicides may cause chlorosis, both to target weeds and occasionally to the crop being treated.
exposure to sulphur dioxide

Of course, the exact conditions would be different for different plants. For example, Azaleas prefer unusually acidic soil and rice isn't troubled by waterlogged soil.

-- Wikipedia

You'd want to try to pinpoint the problem before adding anything. Cold soil or a high pH can prevent the plant from absorbing a nutrient that the soil has no shortage of.

If the NEW leaves are pale with ONLY the main veins dark green, it is iron deficiency.

These symptoms often occur early in the season and then clear up, because, for some reason, roses have trouble taking up iron from cold, wet soils. So there is no urgent need to do anything.

I wouldn't bother to buy an iron product. There is almost always plenty of iron in the soil. The problem is likely caused by a combination of pH above 6.8 and cold, wet soil. However, fertilizing with a soluble such as Miracle Gro for Roses or Miracid will supply available iron.

To reduce the recurrence of symptoms, check the pH and apply sulfur to get a pH of 6.0-6.5. It takes about a year to react fully on the soil..

Thank you, very helpful. It has been a cold though gorgeous spring here in Central PA.