Eastern Rainbow Inc., a graphics services company based in Derry, N.H., and Souhegan Color Inc., a commercial printer in Nashua, N.H., have announced plans to merge their organizations to form the largest commercial printing company in New Hampshire. The combined firms will employ 125 people producing annual sales of $15 million.

Eastern Rainbow president Bob Stuart Jr. estimates that 25% of his business currently comes from supplying design, composition and film services for educational books.

Stuart predicted that efficiencies gained through combining sales forces will increase the combined revenues by some 20% by 2000.