This is a display issue: you want to display a continuous band using categories. You do not need to split your image to create a new new image: this can be done directly on the multiple band image, and you can add the multiple band layer multiple times on the map.
Go to layer properties > Symbology
Select singleband pseudocolor
Choose the band that you want to display
Select a color ramp
Select an interpolation method (discrete is OK)
Select a mode (I suggest quantile)
Select a number of classes
If you want to change some colors, double clic on the color. And if you want to change a threshol value, double clic on the threshold value.
As a remark, your band 6 doesn't look like the NDVI that you could derive from your image. It is more like some interpolated soil properties (or smoothed NDVI, but if you have a NIR band you could have a more precise one.)
EDIT: I now see from one of your comments that you don't use QGIS 3. In QGIS 2, this would be similar except that you must select "style" in the layer properties.