There is a tool in Draw called interactive blend tool.  I assume this is what you have in mind for Paint???  Anyway to answer your question, there is a similar feature in Paint that does almost the same effect, but, it works in a different manner.
With Draw, you are taking two seperate objects (a square and a circle; two different colored squares; a square, a triangle, and a circle - whatever) and combing together the properties of those objects (shape and color) into one object.  In paint you have an "Interactive fill tool" (fly out the paint bucket (Version 9))  This allows you to take an object (or masked area) and select a starting color and blend the colors (along some spectrum - see below) to acheive a gradient color change from one color to the next.  THis can probably be accomplished with multiple objects as well.  As an example fill or paint an area with the starting color (say yellow).  Now select the interactive fill tool and pick a new color (blue).  Click somewhere in the object, and drag the tool in the direction you want your blend.  In the task bar (top) you have some drop down menus (it is different in older versions - if I remember it is all menu driven inside the flyout control) select how you want the blend to occur.  A linear blend will go straight from blue to whatever color is selected (i.e. "to paper color" will go from blue to probably white.  "to transparent" will go from blue to yellow (as that was the color of the object to start with))  If you select other methods of blend, you can see their results.  THe "clockwise through spectrum" was much more apparent in older versions.  It takes a color wheel and blends all of the colors it has to pass through from start color to finish color.  Often times this makes a rainbow effect.  Counterclockwise goes the other direction around the wheel.  Often times one direction is shorter than another, and passes through different colors, so the results are varied.

HTH/  Do some checking on the help features of the interactive fill tool in paint for further explanations.

You may have to blend your boxes in draw, then import them to paint to acheive the effect you are looking for.

Russell