That's a reflection nebula.
Since blue light scatters much more efficiently than yellow and red light, reflection nebulae are usually blue. But not always, because it also depends on the color of the star whose light is being scattered. The white-looking star at bottom right of the picture of IC 4601 is a K0III star, a so-called orange giant, and it emits proportionally much less blue light than the Sun. Its yellow light doesn't scatter so efficently, but there are copious amounts of it. The small amount of blue light it emits is scattered very efficently, but since there isn't much of it to scatter in the first place, the color of the reflection nebulae will be white. By contrast, the stars surrounded by the blue reflection nebulae are of spectral class B and A. Therefore they contain much more blue light than the Sun, and this blue light will scatter very efficiently, making their reflection nebuale very blue.
Take a look at these colorful nebulae surrounding the Antares and Rho Ophiuchi region:
At bottom left of this image, you can see "red" (really orange-yellow) supergiant Antares. It emits extremely little blue light and copious amounts of yellow, orange and red light, so its reflected light will look yellow (since yellow light will scatter much more efficently than orange, let alone red light). At right is a blue star with a remarkable "wall" of pink light on its right side. This "wall" is made of hydrogen which has been "piled up" because of the strong stellar wind from the hot blue star of spectral class O. The pink "wall" glows reddish because of hydrogen emission, so it's an emission nebula. At top is Rho Ophiuchi itself, a multiple star of spectral class B, and it is surrounded by a very large and very blue reflection nebula. Rho Ophiuchi emits copious amounts of blue light which will scatter very efficiently, but these stars are not hot enough to make the surrounding hydrogen glow pink from emission.
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