What do the colours mean in the TortoiseGit log window?

From the TortoiseGit source for Colors.cpp:

• CurrentBranch: Red
• LocalBranch: Green
• RemoteBranch: Peach
• Tag: Yellow
• Stash: Gray

The colors of the branch lines on the left aren't significant and don't correspond to the branch bar colors:

• BranchLine1: Black
• BranchLine2: Red
• BranchLine3: Green
• BranchLine4: Blue
• BranchLine5: Gray
• BranchLine6: Olive
• BranchLine7: Teal
• BranchLine8: Purple

If you are talking about the log dialog:

• the red are the local current branches
• the flesh ones are the remote branches
• the green ones are for tags non-current local branches
• the yellow ones are for tags

Oran's answer (upvoted) has the complete list of color codes.