Splunk Charts applies series colors to all charts. In fact if the series names are same, that is how all series will pick same color.
If you notice in your screenshot, when you choose Categorical colors for the Timeline visualization, the Legends in the same sequence have the same colors as compared to other charts i.e. First color in your timechart as well TimeLine visualization is blue(0x1e93c6) and second is orange(0xf2b827)... and so on.
Following are default series colors picked up by Splunk charts (charting.seriesColors😞
[0x1e93c6, 0xf2b827, 0xd6563c, 0x6a5c9e, 0x31a35f, 0xed8440, 0x3863a0, 0xa2cc3e, 0xcc5068, 0x73427f, 0x11a88b, 0xea9600, 0x0e776d, 0xffb380, 0xaa3977, 0x91af27, 0x4453aa, 0x99712b, 0x553577, 0x97bc71, 0xd35c2d, 0x314d5b, 0x99962b, 0x844539, 0x00b290, 0xe2c188, 0xa34a41, 0x44416d, 0xe29847, 0x8c8910, 0x0b416d, 0x774772, 0x3d9988, 0xbdbd5e, 0x5f7396, 0x844539]
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