PURPLE SWEET POTATO YOGHURT COTTONY CAKE [OGURA CAKE]

I have baked cottony cake with purple sweet potato before [here] but this one here is with yoghurt instead of milk with some minor changes to other ingredients.  The cake turned out well too, the texture is as lovely, spongy and moist but the colour is not purplish at all.
These photos do not look good cos' these are taken in the early morning when the lighting isn't that good.

Ingredients

[8 inch square baking tin]

5 egg yolks

1 egg

50 ml corn oil 

85 gm plain yoghurt

65 gm self raising flour  - sifted - set aside

50 gm mashed sweet potato [purple sweet potato]

1/4 tsp salt

5 egg whites

70 gm sugar

1/2 tsp cream of tartar

1. Line base of a 8" square tin with greased proof paper.   I greased the sides, too and sprinkle lightly with a little flour [this way, the cake can be easily removed from pan after cooling].
2. Preheat oven to 160 degrees C.    Place a baking tray on the lowest rack, pour in enough water [about 1.5 cups] for steam baking, then place a baking rack over it. 
3. Mix yoghurt and mashed sweet potato together until well combined.
4. Whisk egg yolks and 1 whole egg together with flour until creamy, add in yoghurt mixture then gradually drizzle in the corn oil.  Whisk until well combined.  Set aside.
5. Use a cake mixer, whisk egg whites until frothy, then add in cream of tartar.  Beat until soft peaks then gradually add in the sugar in 3 batches and whisk until stiff peaks formed.  Do not over beat egg whites otherwise it may be difficult to fold in to egg yolk mixture.
6. Fold in 1/3 of the meringue into the egg yolk mixture until combined.   Then fold in the rest of the meringue lightly in two portions until well combined. 
7. Pour the batter into prepared cake tin.   Shake the pan slightly to smooth the batter and tap baking tin on work surface a few times to release trapped air bubbles before baking.
8. Place it over the baking rack to steam bake cake in a preheated oven at 160 degrees C for 10 minutes, reduce heat to 150 degrees C and continue to bake for 50 minutes [by then the water bath would have dried up].  
9. Remove cake and invert it to cool for about 5 minutes [otherwise the cake base may be moist because of vapour from steam-baking].  
10. Remove baking tin and the paper linings immediately and leave cake to cool on a wire rack [base facing upwards].  The cake may shrink a little when cooled because of less flour used [low gluten cake].
11. Turn over the cake when completely cooled.  Refrigerate before slicing if preferred.