This thread is for those who like coloring and want to avoid finned sealife and turbot fish whenever possible. It's probably not something especially new, but it might deserve its own thread.
Ruud's Daily Nightmare for April 12 has several interesting coloring opportunities that get progressively more difficult. Basic pre-coloring techniques get you here...
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*-----------*
|..8|6..|..9|
|...|.4.|...|
|.9.|8.5|..1|
|---+---+---|
|...|..6|..2|
|.39|...|16.|
|8..|7..|...|
|---+---+---|
|7..|9.1|.3.|
|...|.2.|...|
|4..|..7|9..|
*-----------*
*-----------------------------------------------------------------------------*
|-1235 12457 8 | 6 137 A23 | 23457 2457 9 |
| 12356 1257 1257 |a123 4 9 | 235678 2578 3568 |
| 236 9 247 | 8 37 5 | 23467 247 1 |
|-------------------------+-------------------------+-------------------------|
| 15 1457 1457 | 135 1359 6 | 3458 4589 2 |
|a25 3 9 |A245 58 48 | 1 6 7 |
| 8 12456 12456 | 7 1359 a23 | 345 459 345 |
|-------------------------+-------------------------+-------------------------|
| 7 2568 256 | 9 568 1 | 24568 3 4568 |
| 9 1568 1356 | 345 2 48 | 5678 1578 568 |
| 4 12568 12356 | 35 3568 7 | 9 1258 568 |
*-----------------------------------------------------------------------------*
...where simple coloring on the 2's eliminates the 2 in r1c1. You can also perform multiple colors on the 3's to eliminate the candidates in r1c7 and r2c4 as shown below....
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*-----------------------------------------------------------------------------*
| 135 12457 8 | 6 137 A23 |-23457 2457 9 |
| 12356 1257 1257 |-123 4 9 | 235678 2578 B3568 |
| 236 9 247 | 8 37 5 | 23467 247 1 |
|-------------------------+-------------------------+-------------------------|
| 15 1457 1457 | 135 1359 6 | 3458 4589 2 |
| 25 3 9 | 245 58 48 | 1 6 7 |
| 8 12456 12456 | 7 1359 a23 | 345 459 b345 |
|-------------------------+-------------------------+-------------------------|
| 7 2568 256 | 9 568 1 | 24568 3 4568 |
| 9 1568 1356 | 345 2 48 | 5678 1578 568 |
| 4 12568 12356 | 35 3568 7 | 9 1258 568 |
*-----------------------------------------------------------------------------*
At this point, you run out of basic coloring options. If we focus on the 2's though...
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*-----------------------------------*
| . 2 . | . . A2 | 2 2 . |
| 2 -2 -2 |a2 . . | 2 2 . |
| 2 . 2 | . . . | 2 2 . |
|-----------+-----------+-----------|
| . . . | . . . | . . 2 |
|a2 . . |A2 . . | . . . |
| . x2 x2 | . . a2 | . . . |
|-----------+-----------+-----------|
| . x2 x2 | . . . | 2 . . |
| . . . | . 2 . | . . . |
| . x2 x2 | . . . | . 2 . |
*-----------------------------------*
...we can note that the only thing preventing a box-box elimination for boxes 4 and 7 which would wipe out all candidate 2's in box 1 columns 2 & 3 is the candidate 2 in r5c1. Since that candidate is colored 'a', we can eliminate all candidates that see both the box-box interaction, and a colored 'a2' candidate. In other words, we have effectively colored the potential box-box interaction. Note that this eliminates the same candidates as the r25c14 finned x-wing.
You can do something similar with the ones....
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*------------------------------------*
| 1 1 . | . A1 . | . . . |
| 1 -1 -1 | a1 . . | . . . |
| . . . | . . . | . . 1 |
|-----------+------------+-----------|
|B1 B1 B1 |bA1 b1 . | . . . |
| . . . | . . . | 1 . . |
| . x1 x1 | . B1 . | . . . |
|-----------+------------+-----------|
| . . . | . . 1 | . . . |
| . x1 x1 | . . . | . 1 . |
| . x1 x1 | . . . | . 1 . |
*------------------------------------*
...Here we do a little grouped multiple colors to show that 'A' excludes 'B', therefore either 'a', 'b', or both are true. Once again we have a potential box-box interaction in boxes 4 & 7 which would wipe out candidates in box 1 columns 2 & 3. The only way to avoid that is if 'B' is true, but if 'B' is true, then 'b' is false, so then 'a' would have to be true as well. Thus, we have effectively colored the box-box interaction 'b', and the 1's can be removed from any cells in box one which see both an 'x' and an 'a'. This replaces a finned r24c14 x-wing.
