Painter Ellsworth Kelly recently turned 90, and to celebrate, the Phillips Collection has mounted an exhibit of six paintings he made between 2004 and 2009. Taken at face value, the pieces don’t seem especially challenging; monochromes never seem to ask for more than an appreciation of color, size, and sometimes shape. But, if viewers are only pondering the paintings’ surfaces, they’re actually not following Kelly’s instructions, written on the entrance wall of the exhibit. “I don’t want you to look at the surface,” the directions say. “I want you to look at the forms, the relationships.”

It’s the relationships between canvases that become interesting. The 2009 work “Purple Curve in Relief” (left) features a purple canvas whose top and sides are flush with a white canvas behind. Only the bottom of the purple canvas is truncated, cutting in an arc against the bottom half of the white canvas. Below the arc, the light draws a shadow against the white canvas. It’s a shadow we might otherwise ignore if it were drawn on a wall. Another work, “Black Diagonal” from 2007 (top), features a long thin black canvas striking diagonally across a shorter, wider white canvas behind it. Black recedes, and white advances, despite the black canvas casting obvious shadows onto the white canvas. If stared at long enough, the black canvas appears to recline in space.

Over his 60-plus years of artmaking, Kelly constantly revisited the interaction between shapes and colors, a fact illuminated by a suite of prints from the museum’s collection, located in a gallery just before the exhibit. While the diagonals of color in most of those prints don’t suggest the dexterity of forms in the exhibit of paintings, one lithograph in particular—-“Yellow/Orange” from 1970—-is significant enough to whet our appetites. On a basic level, it relies on the simple spatial play of isometric drawing; an orange square, rotated on a corner, abuts a yellow chevron. It’s easy to mull over how the two forms relate; is one on top of the other? Inside the other?

By no means is Kelly breaking ground with his suite of paintings. It’s ground he has tread many times, but it remains fertile. And despite their simplicity, the works require a slower read.

“Ellsworth Kelly: Panel Paintings 2004-2009” is on view at the Phillips Collection to Sept. 22.