Dimension line colours in modeling are simply controlled by the colour scheme. Go to Tools, Application Options, Colors tab. The graphic there will illustrate the dimension colour as well as various other item types.
If you don't like any of these combinations, then you can use the Color Scheme Editor to make additional changes. This is a separate tool which you would need to install, which essentially provides a basic Ui to change pre-stored registry values. You can get more info about the editor from "C:\Users\Public\Documents\Autodesk\Inventor 2015\SDK\SDK_Readme.htm" & the usertools.msi file.
Hope this helps,
Chris
Hi yuri.kozak,
Typically we do not worry about the color of the model dimensions since we don't use those for the final documentation. Instead we would create a drawing of the model and dimension it in the drawing. The drawing dimensions can be placed on layers of any color you choose.
http://help.autodesk.com/view/INVNTOR/2014/ENU/?guid=GUID-374B6CC8-B839-4450-8172-2B1174DE9E7D
I hope this helps.
Best of luck to you in all of your Inventor pursuits,
Curtis
http://inventortrenches.blogspot.com