Updated Color to match DataSources and DSName for single-series graphs.

In handling [Bug 1065338](https://bugzilla.redhat.com/show_bug.cgi?id=1065338) in Fedora, we noticed that 6c1415d and 147690b didn't quite migrate everything from v4 to v5. This patch makes the Color entries in collection.conf match the DataSources and DSName when there is only one data series named "value". Without this, the graphs for these types are drawn in gray instead of the intended colors.

I guess this should be merged into collectd-5.4 and trunk as well, since the bug exists from 5.3 onwards.