please try this
void ChangeAxisTitleColor()
{
GraphLayer gl = Project.ActiveLayer();
if( !gl )
return;
Tree trFormat;
trFormat.Root.Axes.X.Titles.BottomTitle.Color.nVal = SYSCOLOR_MAGENTA;
if( 0 == gl.UpdateThemeIDs( trFormat.Root ) )
gl.ApplyFormat( trFormat, true, true, true );
}