For example, you want to set 40% alpha transparence to #000000 (black color), you need to add 66 like this #66000000.
It's possible to use create an MCP installation for versions of Minecraft where there hasn't been a full MCP release. It takes a little bit of manual setup, but the end result is highly useful.
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Download and extract the most recent MCP build from http://www.modcoderpack.com/. (Currently, the latest build is http://www.modcoderpack.com/files/mcp940.zip) 
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Edit version.cfgin theconffolder, and changeClientVersionandServerVersionto the version you want (for instance, 1.12.2).
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Download the SRG zip for the version you want; these can generally be found at http://mcpbot.bspk.rs/mcp/<version>/mcp-<version>-srg.zip(for example, http://mcpbot.bspk.rs/mcp/1.11.2/mcp-1.11.2-srg.zip) or athttp://files.minecraftforge.net/maven/de/oceanlabs/mcp/mcp/<version>/mcp-<version>-srg.zip(for example, http://files.minecraftforge.net/maven/de/oceanlabs/mcp/mcp/1.12.2/mcp-1.12.2-srg.zip). (For 1.12.1 and 1.12.2, only the minecraftforge link works)
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Extract that zip into the MCP conffolder, over