Can I use an 18-bit RGB screen with EVE devices which have 24-bit output?

Yes, this is possible. The FT812/3 and the BT81x series have 24-bit outputs on their RGB interface (8 red, 8 green and 8 blue). However, they can be used with LCD panels which have a smaller color depth. Many displays have 18-bit RGB interfaces (6 lines per color). When using a display with fewer bits per color, the REG_OUTBITS register can be written to set the number of lines per color. See the EVE Programmers Guide for details of this register. When setting a number of bits which is less than the maximum for your EVE family device (for example setting a BT817 to 6 bits per color instead of 8) note that the output is justified to the upper bits. Therefore, your display’s R5:0, G5:0 and B5:0 would be connected to EVE output pins R7:2, G7:2 and B7:2 respectively. The output lines R1, R0, G1, G0, B1, B0 on EVE would not be used.