Watercolor (or Colourwash) quilts are a relatively recent innovation in quilting.  Here the challenge is not fitting complex shapes, rather the challenges are in "painting" an image then assembling up to 4,000 squares two-by-two.  Bonny Tinling developed the Squares on Fusible Interfacing technique.  

The basic method (To make a typical watercolor scene or counted cross stitch piece)

Arrange squares on a large grid of fusible interfacing. Fuse in place.

Fold and stitch rows in one direction only.

Snip all intersections to slightly past the line of stitches.

Stitch crosswise rows.  Keep seam allowances in opposite directions to make butted intersections..

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A great variation (Especially good for larger quilts where patch-by-patch design is not practical.  ) 

In the book Watercolor: An Easy Approach, Bonny has taken the next step by making even very large watercolor quilts quite practical ... she has broken the quilt down into very manageable 25-patch blocks assembled using gridded interfacing.  Because all blocks are alike, design is much quicker and you may use yardage (9 or 25 fabrics) instead of accumulating thousands of scraps.

Here's how it is made: 

Cut fabrics into 2" squares.  Cut interfacing (2" grid) into 12" x 12" sections -- note these have 5 rows of 5 squares surrounded by a 1" border of half squares. 

 Arrange 25 pieces on interfacing shading darkest to lightest.  (Fusible side of interfacing faces up.)

Fuse using a non-stick appliqué pressing sheet to protect iron. 

Choose a block arrangement.

Trim excess interfacing and build rows by overlapping edges of blocks.

Stitch rows in one direction only.

Snip all intersections.

Stitch seams that run in the opposite direction.  Take care to butt seam allowances.

Add borders, quilt, and bind.

Related Products

Watercolor - An Easy Approach Book

Complete instructions including yardages for nine sizes.  Uses 2" grid shown below.

 
Watercolor Panel

Quiltsmart's gridded interfacing is available in several sizes:  1", 1-1/2", 2" (shown here), 2" On Point, and 2-1/2". 

Each 36" x 24" panel yields 6 25-patch blocks

Panels required by quilt size: