Yellow is a primary color. There's no way to darken it without going cool (blue) or warm (red). It's likely that your yellow is already tilted towards one or the other of these ranges. First, determine what range your yellow falls in. If it's a cool yellow (heading into the green range) then adding a touch of violet will darken it without shifting it's shade too much. If it's a warm yellow (getting a little orange) then a little bit if umber should do the trick. Simply adding black doesn't give the best results because it just muddies up the color. Your goal is to deepen the shade while keeping it's sparkle up.
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