In fixtures that use multiple bulbs, you could have a mixture of cool-white LEDs and incandescents, which makes what I think is very pleasant light. Then you could obtain (at leisure, and waiting for the best price) a supply of warm-white LEDs and gradually replace all the incandescents.
Essentially all white LEDs actually are three-LED combinations -- red, green, blue. Typically each has a separate dropping resistor, and the manufacturer can vary the resistances to obtain any color balance from very blue to very red, or anything in between. The only color they can't get yet is purple, for there are no purple LEDs. Several years ago, but not that long ago, there were no blue LEDs either, so they couldn't mix for white.
Someone mentioned 5050, 3528, and 1210 LEDs. Those numbers indicate the size of the SMD chip that contains the three LEDs, or only one sometimes. 5050 LEDs are 5.0 mm square, 3528 LEDs are 3.5 x 2.8 mm, etc. The larger chips have more external connections, enabling more versatile control. Also they have better heat transfer to the substrate and can run a little more current, so often they are brighter (depending on the choice of resistors).
5050 white (tricolor) LEDs have six external connections, two for each colored LED, so a designer can wire them any way he wants. For example, if he wants to run the same current through the G and B but a different current through the R, he can put the G and B diodes in series and use only two resistors (one for the G and B, another for the R).
3528 tricolor LEDs bring the positive terminals out separately on three external connections and tie the negatives together on a fourth (or vice versa). They require separate resistors for each of the three diodes, because if you put two or three on a single resistor there's no telling how they'll share the current; usually one will have a slightly smaller drop and hog most or all of the current.
1210 LEDs are single-color LEDs. A designer has to use at least three different colors, sometimes four (RYGB) to mix for white light. A single 5050 may easily be as bright as three 1210s. So, if you're looking at incandescent-replacing white LED assemblies, one with 30 1210s won't be nearly as bright as one with 30 5050s or 3528s. Buyer beware.
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-- Carl
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