I believe that you can actually achieve this... but, the method that sticks out in my head is a little complex since it basically means you have to extract the RGBA values, store them as floats, do the appropriate maths on them (average, aka, add them together, divide by two) and then use the resulting values to recombine into an RGBA value you can then feed into a colour action.
the pertinent actions you'd want to use for the colour values would be the Get/Set Color RGBA actions in the Color subsection (*chuckles* kinda feels odd for me spelling it like that... *shrugs* but what can i say, i'm a crazy canadian
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So, maybe state one will have an idle waiting for you to tell it to go... then the next state will extract the value from the colour variables you want (two different variables in your variables tab) then extracting the RGBA into floats (you'd need to declare eight of them... four for each colour variable) do the maths (add together then divide by two) and then recombine them in the next state and do what you need there.
As for a system to determine the amount of blending... well, i'd have to do a bit of testing before i'd comment on that specifically but i'm sure it's possible... but, the maths involved aren't really coming to me very well (i'm an artist, not a brain. :lol:)
... so, i guess i'm just going to spit-ball here.
So, you have a value you want to blend... so, a from 0-1. I don't know how you're going to interact with it but that should not be hard to do (i'm also assuming you already have a system to control the value of the blend.)
so, from there, i'd probably want to take the values of the colours... once they're separated into individual floats, maybe multiply values of the first colours by the blend amount... and multiply the second colours by the opposite (so, take the blend value and subtract it from 1) and then combine the two together? (maybe averaging, i'm not sure... this is where my math knowledge is getting really shaky.) i'm sure this would be a really beefy state to be in so maybe if the blend needs to change, have an idle state detect when that float value has changed, then run through the algorithm, send off the results, then return to the idle state (i'd probably want to add in a microsecond pause or a next-frame-event... but i'm finicky/OCD like that and that might be completely extraneous to what you'd need.)
if anyone is better at the maths here, please, please let me know if i'm getting it wrong. I suspect I am but as i said before, i'm an artist, not a brain.
Also... did some testing... the colour get/set actions work with values from 0-1 as well... so, the maths should still hold up for extracting and comparing... but it might be a head-trip converting the typical 256 value that most people think of when they think of colour values in computer systems.