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You can absolutely wear your tan purse with your brown sandals, though your hesitancy is understandable. There was a time when it was considered tacky to wear shoes that didn't match your bag. However, as time evolves fashion evolves as well, as do those old, antiquated fashion rules.
If you need proof just look up runway images or red carpet celeb photos. Unique color combinations are actually very in style right now. This means that you can wear any shade of brown bag with any other shade of brown shoe and be just fine.
Just because your brown bag doesn't have to be worn with your brown purse doesn't mean that it's still not possible to look completely mismatched and wrong. Try, instead, pairing complementary colors. For instance, not only can your tan bag be worn with brown shoes, it would also look great with metallic colored shoes, and could even be worn with a brighter color like red.
How Far is Too Far
Really, almost anything goes. Fashion insiders have mixed black and brown, which in the past was a huge faux pas. As long as your colors don't totally clash, you should be just fine.
If you're still not completely sure whether you match, try checking out a rudimentary color wheel. You can find a good one in the Resources section. The color wheel will give you a basic idea of what colors traditionally work together. Also, know that if you just prefer to match your shoes to your bag, that's OK too. Most importantly, remember that style is fun. Embrace your style and if you like it, wear it!