The left is a pottery being processed.
The top is the pottery, the bottom 土 is character Soil (Post 223), which means that people make the soil to form a certain shape of pottery. Another explanation could be the base frame of pottery making, on which the soil is put on to be processed.
The right is the light shining on the pottery, which makes the pottery show the shape much better.
The above is my explanation.
According Dictionary of Pictography (象形字典), the left part is a Well (Post 59) - colored clay mine. The right part is the colorful lines people drew on the pottery. I don't agree with the explanation because the left part in other characters means "pottery making process", and right part is light in all other characters, one example is 影 - shadow (Post 237). More important, adding color to pottery has nothing to do with its shape.
Because there is no ancient documents exist for the explanation of what the real meaning original, so many explanations can't be verified, or proved to be true. Different people may have different explanations. You can choose which one you feel the most rational to you.
The another big seal script for the character is
The pottery shape changed to two 干s, the right one has the three strokes attached. I don't know why the shape of pottery changed to two 干s.
The small seal script for the character Shape is
Its Pinyin is Xing1.