Updated the default value in the unit test

Fixes this regression reported by Jenkins:

Error Message

expected: but was:
Stacktrace

java.lang.AssertionError: expected: but was:
at org.junit.Assert.fail(Assert.java:88)
at org.junit.Assert.failNotEquals(Assert.java:743)
at org.junit.Assert.assertEquals(Assert.java:118)
at org.junit.Assert.assertEquals(Assert.java:144)
at org.openscience.cdk.renderer.color.CDK2DAtomColorsTest.testGetDefaultAtomColor(CDK2DAtomColorsTest.java:55)