Customize Colormap for discret values

Hello,

I need to build a a "really" customized color map.

I have a matrix with discret entrys from -1 to 30.

-1 is the default variable and should be displayed in white. 29,30 should be grey. the rest could be continous like the existing colormaps or also be set by value.

If searched online but I couldn't get an answer. Is there even a function like that?

Thanks a lot in advance.