What I've always heard is that the intention from the start was for the suit to look roughly similar to how it ended up looking in Robocop 2, and that the first suit was indeed painted with a similar amount of color (which you can easily see in behind the scenes footage). However the methods and paints they used ended up appearing much more subtle than intended on film, depending on the lighting of the scene. Sometimes you can see the colors well, other times the suit appears silver (fun side-fact, the special metallic paint they used looked greenish in real life, though not on camera).
They rectified this for the sequel with different paints that made the color much more obvious and pronounced on film, which is what gives viewers the general impression that Robo was silver in the first movie and more blue in the second (even though reality is not as cut and dry as that).
Both of these shots are from Robocop 1 and yet one looks silver and the other looks blue:
Meanwhile for Robo 2, his appearance remains relatively consistent across lighting conditions due to the different and more pronounced paints:
As for the rushed trash heap that was Robocop 3 (that's so creatively detached from the first two that I don't even consider it a canon sequel, just a studio cash grab), they tried to save money by reusing suit parts & molds remaining from R2 by choosing an actor that could fit into them with minimal modifications. Whether it be from the way they were shot, or from having both a reused R2 suit and perhaps newly fabricated suits during production (there is always more than one suit during filming), the colors again appeared inconsistent. Whatever the reason, this leads to the perception that R3 is again more silver than R2: