See Harold's gold washing procedures in the thread getting gold pure and shining, this will help to turn the dark brown powder to a light brown (tan) powder (removing the base metals that darken the powder).
Re-refining can also remove impurity. Refining the second time is similar to re-crystallizing salts to purify them, and is normally fairly easy to do the second time.
The light brown powder is gold, if you crushed it with a glass stir rod it would look like gold, (or smeared in under pressure onto a piece of paper it would leave a gold streak), the pure powder will do this where impure gold will not do this or as well.
To get the brown powder look like gold it is melted, it has something to do with particle size and reflection of light as to why this fairly pure brown power does not look like gold.
I do not know how you could change the particle size or form that the brown gold precipitate to make it to look like gold power, without pressure to smash the fine powders together into metal, or by melting the powders back to a lump of gold metal, both of which would join the powders together into another shape, I have seen when melting just before the gold melts the powders join together under the torch and begin to look like gold, I have assume this was the fine powder melting and beginning to melt together.
Gold can be precipitated using oxalic acid, on the second refine.
The second refine (using the different reagent) will improve gold quality as the different reagent will not have the drag down or precipitation of metals the first reagent used will include in the precipitated gold.http://www.youtube.com/watch?v=o165tgxFMYM
The carbon based reagent's used to precipitate gold seem to precipitate gold with more of the golden color.