fix: color bitmask calculation

[jetjinser]

Consider the following code: let mask = !(0xF0 >> (pos % 2));

In this code, 0xF0 represents the binary value 0b11110000. When pos is even, (pos % 2) equals 0. Therefore, the right shift operation on 0b11110000 by 0 does not change the value, resulting in mask = 0b00001111.
However, when pos is odd, (pos % 2) equals 1. In this case, the right shift operation on 0b11110000 by 1 results in 0b01111000, which is incorrect.

To fix this issue, modify the code to let mask = !(0xF0 >> (pos % 2) * 4);.

With this modification, when pos is even, (pos % 2) equals 0, and the right shift operation by 0 * 4 = 0 does not change the value, resulting in mask = 0b00001111.

Similarly, when pos is odd, (pos % 2) equals 1, and the right shift operation by 1 * 4 = 4 correctly results in 0b00001111, giving mask = 0b11110000.