[Feature] not to reassign series colors

### What problem does this feature solve?

We start with some series with default colors (unassigned).
When one series color is programmatically set, the other series colors auto-update - [Demo](https://echarts.apache.org/examples/en/editor.html?c=bar-y-category-stack&code=PYBwLglsB2AEC8sDeAoWszGAG0iAXMmuhgE4QDmFApqYQOQCGAHhAM70A0x6L7ACsAjQwtQqhIkA9FNgBVNtVh82GYGUo1SanHh4kwATxDUGbABaMAJsADu9WDNj0L1uw8aqr1AGaMArrgA3LAAxoxwjNhs6gBGSvTYwtQOwNouljb2-gC-xDnc6JBg2KZIosxg-PShSaEA1rAwyrCxjNqK5NSqALQAfE1g5rSwnRDdYThpqu1KpNSebJTQ1Fb0BcSlNNBW4q3AYJgAtvgATKcb6MwAgqxse5ewhrfs4vpGJgzhohRphlz6KyMMCMQgAbXoAFkYFxnAAVfwpTjOADqq1h9Dh5n8GIAYuQMQBlYFE_zQegAXXyhVGtHG91gYP0EkksGgjCOpmcABEIPNQmAAayMMYufQ2qQhay2CCGgxMCDsFLJNhGPFsG9hegLHZCGBSIj9OhHpIgSDwQBmU4ABmRFutpzt1oAjHaLQAWO0ATltsAt9rtNqpkhNRGF7M5DEhjAg2Fg1zWNMkHzFEuVJBljDlzgVUXT6FV6s1Wp1tj1BuoRtgofQZtBjOdNuRzqtzZdzY9yJ9yNOAdgp2d1uDJFDLMkEbF1x8PljEGBSgT-ZFn2caaTGdl9XlBzz64LauoGrDJfMurIhuFNdgdfB519zoAHI7YM6va7-53-92_X2LYPh8a65jiQE4MAAahA3jqIue7Lqm7RLpm2b0LmSqwYWh7FsKpblherJXjeDYAKy-gOz42u-zpEZ6L7fv6vruv-uRAVWoHOISCykKE5iwAAotAFDJEuKYMGuVZIVuOY7mhVYYUewGSDh56VpesGEWCT6-g-rawD675ep-jbfi2fYtkGuTEFSORBCgRyGAAwpYpBgAAdDAAAUNR1PUsLMAgAxILAxCgJAMAuWM3Rgsw4V0t0ACSOzUMwFIuRAohHISRilIgSChFMdDiqqDTrDZ6B2Y57SuYoYAAPLgFA0DuSFDUAJQ2TkLVAA).
It seems more like a bug to me, but lets call it a Feature since no one has complained about it yet.

### What does the proposed API look like?

Make sure not to alter other series when **only one** is updated. It creates confusion in the chart.

Start with the linked bar-y-category-stack demo and reproduce the behavior where setting one series color changes the others. Trace the color assignment behavior from that example, then verify that updating only one series leaves all other series colors unchanged.

Written by the indexing model from the issue text.