<p>What does the pink color in Slic3r preview mean? Yellow is my model, green is support, and pink is..?</p>

<p>If the pink color is some kind of warning, how do I fix it?</p>

<p><a href="https://i.stack.imgur.com/2PIRU.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/2PIRU.jpg" alt="Illustration from preview"></a></p>

Having had direct experience with Slic3r, I can offer up this information. Your model is composed of bottom layers, top layers, outside layers, infill, rafts, brims and perhaps something I've missed.
The program provides for color coding of these features. In the case of your image, the pink represents a top layer, but may also represent a type of infill, depending on "context."
Consider to slice the model, select the preview tab, which you have showing here, then using the slider control to the right of the image window. As you move it from bottom to top, you can observe the construction of the model and each feature as it appears, layer by layer.