...and thank you for the responses.But I'm still unsure about how to even out color. I now better understand that the problem is that they used sapwood, which is of course yellower. How do I battle that? Just use a cooler dark brown all over and hope it evens out with age (I did that on a piece of cherry and after several years there isnt that much of a difference between the heart and sapwood). ...and the question about the gaps between the boards: how do I handle what is plainly their poor joinery? Can I just wood putty it? Seems the only option...
Regarding the gaps, what sort of pieces do you have? Tables? Chairs? Are the gaps at joints or between boards that form a surface? If the latter, are the boards actually meant to abut or is there a reason for the gap?
Regarding wood putty, what kind of wood putty were you thinking of using? Regardless of type, it would mosy likely look bloody awful.
Regarding heart/sap wood color disparity...it is not uncommon for some woods to be fumed with ammonia to even up the colors. For it to work, the wood needs to have tannin in it; IIRC, teak dies; if it doesn't, it can be added (tea is high in tannic acid). Howeveer, fuming is not something you should undertake.
Time - along with exposure to light and air - will naturally even up the colors to a considerable extent. I wouldn't suggest staining especially since I haven't a clue as to what type of piece(s) you have or if it (they) are to be inside or outside.
It seems to me you have three options...
Learn to like what you have with the expectation that time will help blend the colors
Take it to a furniture restorer - not a carpenter, not a cabinet shop, a furniture restorer - and be prepared to pay dearly.
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