Try to clear one of the colors off the screen by exploding blobs.
Grow the blobs until they pop, sending bloblets around. A blob will pop when it has size 4, sending a bloblet in each direction.
Bloblets fly past other colored blobs. They merge with the same color. They land on an empty square. If they go off screen you collect it into your inventory.
Select a color from your inventory (the numbers at the bottom). Click on a blob of that color to add your bloblet to it.