The i.MX51 or Wi-i.MX51 module provides 24bit color on pins J2:1-24. If you connect two LCDs in dual display mode the color on each LCD is reduced to 18Bit. Thats why on the Digi JSK (GA) board the LCD1 and LCD2 connectors provide only 18bit color.
However you can use 24bit color in single display mode on your custom board or with the Digi EA-Kit board (LCD1 connector).
In the Digi Linux ESP DEL5.6 select a Linux kernel project. Open the project Linux kernel configuration. Select System Type, Freescale MXC Implementations. Select development board variant, select Connecte Core ... Early Availability (JSCCWMX51_V1).
If you now select Video Interface(s), the Display 1 color mode will have automatically switched to 24 bit (CCWMX51_DISP0_RGB888).