Interesting that the subject of anger comes immediately after dealing with telling the truth. As I suggested yesterday, though we are to speak truth, we should do it with love and "cushion" our constructive criticisms that they are not delivered with any sense of anger.
Other versions say, "Be angry and sin not..." Does that mean that there is a "right time" to be angry? Think about it. What SHOULD a Christian be angry about? To me, the most anger I should exhibit is against my own sins! If there is anyone who I feel I should express anger to, it's me! If I see a brother or sister living in a fault, I should be angry at their sin, but not at them. Hard to do? Yep, but the Holy Spirit can control our emotions to help us demonstrate tough love when we need to.
What about "sunset anger?" What Paul is stressing is that we should settle the issue in our own hearts, for it may not be possible to reach the person we got upset with before bedtime! Surely, in our homes we should apologize either for our sin, or our outburst of anger that compounded the stress. Sure, it would be great to settle the disagreements quickly, but sometimes we've learned we have to "disagree agreeably."
"A foothold to the devil." That's something we must avoid. Dr. Bob Cook, former president of Kings College, used to say, "Don't give the devil a handle in your life to use against you." Anger can provide just that handle. Remember Proverbs 15:1, "A gentle answer deflects anger." Be gentle.