Colour can make or break a doily. A doily that makes good use of negative space can be really beautiful and a designer may use colour to emphasize the design and make it really "pop". Think of a simple ring and chain edging. You can work it all in one colour or you can use one colour for rings and another for chains. Do the ring in red and the chain in green and you go from a ring and chain edging to a row of flowers. Similarly colour in a doily can be used to emphasize one element while another is subdued.
See this pink butterfly? The first one was done all in one colour and while you could tell it was a butterfly, it didn't really come to life until I changed the middle row of split rings to black. It separated both sides and defined the body of the butterfly.
Variegated threads can add a whole new dimension to tatting and the effects can be terrific or terrible. This is the same butterfly using both solid and variegated thread, but the placement of the light and dark sections of the thread was deliberate. The butterfly is made using 2 shuttles. One was wound with the solid blue that is a pale shade matching the palest colour of the variegated thread and the other shuttle was wound with the variegated thread. The darkest sections were used close to the butterfly body which is black just like the pink butterfly. The colour changes from dark to light as it works toward the outer edge of the butterfly wings. You might think that the whole butterfly was done in just variegated thread, but it wasn't.
Sometimes variegated thread can really add a wonderful dimension to tatting and sometimes it can completely ruin a beautiful design. In a doily where very effective use of negative space has been made so that shapes are evident within the design it is best to use a solid colour thread. A variegated thread will usually draw the eye to the colour and distract from the shape. In that instance colour and line would be at war with one another and neither win.
In a doily where the line and shape are simple and repetitive, variegated thread can break up the monotony and bring new life into the pattern. Long stretches of colour with gradual changes can make one effect on a design while another thread with short quick colour changes can bring a totally different effect. Sometimes the short quick changes can make a finished piece look choppy, but if the design warrants that kind of quick colour changes, it can be a very useful design tool.