The root cause of the CWE-125 (Out-of-bounds Read) vulnerability in this code slice is:

1. Lack of bounds checking: The code directly accesses the colormap array using the value of 't' without verifying if 't' is within the valid range of indices for the colormap.

2. Potential for invalid input: The value of 't' is obtained from the input data (*inptr) without any validation. If the input data contains values outside the expected range, it could lead to accessing memory beyond the bounds of the colormap array.

3. Assumption about input data: The code assumes that all values in the input data will correspond to valid indices in the colormap, which may not always be true, especially if the input is untrusted or corrupted.

4. Missing information about array size: The code comment mentions that "colormap[x] has size cmaplen", but this length is not used to perform any bounds checking.

5. Direct use of input as array index: The value 't' obtained from the input is directly used as an index without any sanitization or range checking.

To fix this vulnerability, proper bounds checking should be implemented before accessing the colormap array. The code should verify that 't' is within the valid range of indices for the colormap (0 to cmaplen-1) before using it to access the array.