Purifying washing soda
If there is an organic contaminant in the $\ce{Na2CO3}$, you could try to rinse the sodium carbonate with an organic solvent in which $\ce{Na2CO3}$ is insoluble.  Based on this paper, acetone and methanol would qualify.  Since those solvents, especially methanol, are toxic, I'm not recommending either one of those.  Based on the data for methanol and acetone, I'm guessing absolute ethanol would also be incapable of dissolving sodium carbonate, so it would probably the best (relatively) non-toxic solvent for extraction.  If you can't get absolute ethanol due to tax or beverage laws, you could probably used denatured ethyl alcohol instead.  It usually has small percentages of methanol, acetone, and/or isopropyl alcohol added to make it non-potable.

After you wash the sodium carbonate in this solvent, you should air dry it for several days, and then dry it in an explosion-proof drying oven at 100 - 200 &deg;C.  Be very careful when combining flammable solvents with heated environments!

The problem could also be with the water you used, although you did say you used osmotically purified water.  The reason I think the water may be the culprit is that if the washing soda contained a brown contaminant, I would think it render the powder visibly brown.  If no brownness is evident until adding the soda to the water, it makes me think the high pH from the washing soda is precipitating minerals from the water, in particular iron.