Boston — Two scraps of colored paper, one blue and one yellow, have captured the imagination of millions of children since author-illustrator Leo Lionni first began to produce children's books, turning out his ``Little Blue and Little Yellow'' a quarter-century ago. This year Pantheon Books is marking his 25th anniversary as an author with the publication of ``Frederick's Fables: A Leo Lionni Treasury of Favorite Stories,'' which includes such admired tales as ``Frederick,'' ``Swimmy,'' and ``Alexander and the W ind-Up Mouse.'' But Mr. Lionni, an internationally renowned graphic designer who earlier this year was given one of that field's most prestigious awards -- the Gold Medal from the American Institute of Graphic Arts -- is as famous for his advertising and design work as for his books for children.
He finds, in fact, that the two careers overlap. He considers himself a designer rather than an illustrator and conceives of his books as a unit. He has made it a point never to illustrate another author's story, because he likes to deal with ``the whole object. If I started doing that,'' he explains, ``my whole work would change, because I would have to invent styles for stories which were not my style.''
The pairing of writing with illustration gives him control over the flow of text and picture. ``I hope to achieve a coherence between the visual style and the verbal style,'' he says in a quiet but firm voice. ``It is really one of my great ambitions.''
Of all the book illustrations Lionni has published, he is proudest of the first page in ``Little Blue and Little Yellow.'' The ``characters'' in this book are blue and yellow pieces of paper, and by positioning his scraps in particular ways, he managed to create a vital book out of them.
His placement of ``Little Blue,'' for example, illustrates what Lionni feels is a strong coherence between the visual and the verbal. ``There are thousands of places you can position that daub, and there are a thousand ways to begin the story,'' he explains, smiling slightly. ``But I think the positioning of the daub in the middle, and saying very plainly and bluntly, `This is Little Blue','' achieves the best balance between what the reader sees and hears.
Picture books and the relations between their visual and verbal components are important for young children, educators have noted. Lionni could not agree more. ``Before a child reads,'' he explains in a lightly accented voice, a picture book gives a child ``a sense of structure of information,'' and it does so ``without him knowing it, in a very original way.'' The structure shows a child that there's a beginning and an end, that actions follow in sequences, and that there is a connection between imager y and words.
He points out that a child also learns how to fill in the ``fictional gap'' -- in other words, what happens between the time a character is shown getting out of a car, say, and then sitting on the front porch. ``In an illustrated book, this happens between the spreads from page to page,'' he says, ``and you get the child in the habit of filling in -- of completing -- the story between the elements that are not said or shown.''
Parents play an important role in a child's reading, Lionni says. ``They're the ones who are the intermediaries . . . -- they read the stories, and they have to act it out. If they read a book they like, it comes across [to] the child. If they don't like the book, they're not going to do a good acting job.''
Although many children's writers try to keep in close touch with today's youngsters so as to reach them at their own level, Lionni's approach is different. He believes that the feelings and motivations of children have changed considerably since he was a child growing up in the Netherlands, Belgium, and Italy some 65 years ago. ``One of the reasons I do animal fables,'' he explains, ``is that that doesn't tie me to any specific kind of children -- a blond or black or brown or red or yellow child. It doe sn't tie me to any specific surroundings, like skyscrapers or hamlets.'' Working with animals gives him the freedom to tackle can-do books about problems, feelings, and concerns that are universal, he says.
The animals in Lionni's books are not elaborately drawn. His aim is to reduce each one to a concise symbol. Mice are some of his favorite characters. ``All the bodies are the same, all the ears are the same, the legs. When I work, I have little dishes before me . . . . [with] different kinds of paper: I have ear paper and film paper and what I call mouse paper, which is the gray paper to make the mice with,'' he says. It is the positioning of the eyes that gives the m ice their individuality, according to Lionni, and the arrangement of the mice on the page that conveys the relationships and moods of the story.
Several of his books have been made into animated films. At the moment, he is working on three new ones for Swiss television. He thinks that the quality of the art is comparable, whichever the medium. But he believes that television cannot teach children to take an active part in what they are doing -- and that books can.
Children, he says, ``participate very little'' while watching television. With a book, a child's imagination is stimulated and he or she can create a fantasy world, he explains. The child can also carry the book around -- in the car, to school, or out shopping. With a television show, once it's over, there's nothing left.
When Lionni was growing up, of course, there was no television. The school he attended emphasized nature studies, and these helped foster what became a lifelong interest. As a child, he furthered this interest with a collection of small animals, mostly reptiles, which he kept in terrariums.
Today Lionni doesn't have time to keep a terrarium, though he wishes he could. Instead, he and his wife have a home in the countryside near Siena, Italy, where animals roam freely through the nearby woods. The Lionnis divide their time between Siena and New York City. And although their New York apartment lacks the natural charms of the Siena home, Lionni likes the urban side of his life too.
``You always think of a city as this big thing that you live in,'' he notes. ``Well, it isn't actually true, because you live in a small part of the city. If you like that part of the city and feel comfortable, you're very fortunate.'' New York ``is there to remind me of the cultural'' side of life.